I think some people misunderstand the OP and some other points of view. I think the point being made is that Metroid should take certain steps if they want the series to have a smash hit, not that it would necessarily be a great game. 2D Metroid and Metroid Prime gameplay are too niche to sell more than 5-7 million copies at best.
If Metroid wants to sell a lot it needs to be more like Doom or other modern shooters, or an open-world adventure like Breath of the Wild. It also needs more voice acting and cutscenes. And no, not like the ones in Metroid: Other M.

Your post still isn't clear.  What it seems to be saying to me is that these elements are there, but they are spread out across several different Metroid games.  That is actually what dilution is.

Ok, I feel it should be clear if someone has some basic knowledge of the series or has played the games to a decent extent but I'll develop each point further, even correct myself.

A) The entire Prime series is T rated, just used the original Prime as the base example. The most dilution one could argue with this is Samus Returns being E10+ but I think is a stretch given how some elements of how ratings get affected have shifted over time.

B) Prime 2, Fusion, and the game I forgot to add Samus Returns (3DS) are the fair, more difficult games of the series, and honestly is not like the rest of the games post-NES lack challenging parts or set pieces, is that these 3 games are the most consistently challenging, and to be honest if the threshold to surpass are the guardians of BotW, the series can pass the difficulty test without much problem.

C) Here I also made a mistake in not including most of the games, is just that Prime 2 is an example that sticks out the most in that premise because that premise is outright stated by the small parts of mandatory story exposition, but the entire Prime trilogy deals with planets where high tech civilizations has been mostly or entirely wiped out, if anyone knows what those games are about Prime 1 deals with the fall of the Chozo at the hands of the Phazon, and 3 also deals with that in its various planets, Fusion deals with the X virus, and the new lore of the Samus Returns remake also has to do with groups being wiped out, and Metroids themselves also play a big part in all these stories, so I stand by these points not being diluted, they are ever present.

D) I don't want to go on about every set piece in the series, but there are always moments where Metroid is unnerving, Fusion is the well renowned peak of this with the SA-X, the whole build up to Nightmare, among others, the whole Space Pirates ship in Zero Mission which on top of being its own unnerving moment is also designed to catch a veteran player out of guard since is an entirely new area of the game, Prime has the visit to the Phazon Mines with continuous attacks from the elite space pirates and very sparse save points, the ghosts Chozo that appear out of nowhere, how the Dark World of Prime 2 is a wasteland that consistently keeps you on the move since your health is always going down outside of safe areas, the constant threat of the Phazon in 3, etc. There are a lot of other moments and sequences designed to unnerve across the series thanks to both its presentation and gameplay, there are just some highlights from the rest of the games.

The only game one could point to a dilution of Metroid elements is Other M.

Now, if you want a serious suggestion, i think metroid prime should be directed by shinji mikami.

what would make it sell 20m is just naming it PS5

in all seriousness though you are removing the essence of what Metroid is which is what made me fall in love with it all the way back on the GC days! I just think it hasn't had the right install base and correct marketing, timing and exposure it needs. It might do 5+ m on switch if it is released with enough time before le Switch 2 cometh out

I actually feel like the rumored 2D Metroid could sell like 5 million itself. 2D retro games are popular these days, lots of pent up demand for Metroid Prime 4 and some of these people will get a 2D Metroid to fill the void for the time being. Though if it comes out at $60 the full price tag may put some people off from buying a 2D game, hopefully it's like $40 unless it's just mind-blowingly amazing. So I think Metroid Prime 4 has a chance to do close to 10 million if they just make it a knockout awesome game that is perhaps a bit more lively in its setting and less of like an abandoned civilization setting. 20 million I don't seeing without Metroid losing what makes it Metroid.

You are picking away at my trees, and yet you are not seeing my forest.  A person in 2016 could have described BotW as what they wanted in a Zelda game, and someone like yourself could have rebutted their points in a similar way.  "I want a huge, freedom focused, open-world game that is challenging.  Make the overworld awesome!"  "Zelda has already done this.  Lots of Zelda games are open world.  Wind Waker has a huge open world.  Previous Zelda games have challenging parts." 

This is missing the point.  Because before BotW none of the games had the freedom of the original Zelda, nor did they have the challenge of the original Zelda.  The details of Zelda 1 and BotW are extremely different.  However the underlying design philosphy is extremely similar: explore a huge world with extreme freedom, even to the point that the player can easily get in over their head and die.  BotW went back to the original philosophy of Zelda 1 while making it look like a modern game in 2017.

Metroid 1's underlying philosophy is somewhat different: open-world space horror, i.e. make the game mess with the player's head however they can.  The isolation, the challenge, getting lost, etc... is all meant to mess with the players head.  Metroid 1 doesn't even have real boss fights.  Ridley and Kraid are areas you can exit at any time.  There is a fake boss.  The final boss of the game, Mother Brain, is just a giant brain in a jar.  All of this is to mess with the player's head.  There are tons of secrets and also some trap floors.  You don't know what you are going to get next.  I beat Metroid 1 without killing a single Metroid.  That's not the way I wanted it, I just didn't know how to kill them.  They felt unkillable.  That all added to the horror feel.

Metroid 1 is a fairly frustrating game.  You know what other game is frustrating? Dark Souls.  Frustrating is not necessarily bad in a horror themed game.  You somewhat need to feel like your situation is hopeless.  I would be happy if Metroid tried to make a more extreme Dark Souls in space.  Make the game harder and more unsettling than any Souls game.

The main problem with the Metroid series is that the details are too similar to previous games.  I can't be disoriented when I know what to expect.  In the first game, I had no idea what to expect.  Now I feel like they keep rehashing most of the same parts while making the game easier and more linear than the original.  I don't want a Metroid game that has the elements, the details, of previous Metroid games.  I want a Metroid game that gets back to the design philosophy of the original: open-world space horror.

To be fair, you did make an entire list of things Metroid should do, then got confronted with the fact that Metroid has done basically all those things. And instead of backing away you retreated to some vague idea of "none of the games capture Metroid 1's ideals", without actually realizing that not only did pretty much every 2D game aside from Metroid Fusion capture the exact ideal you are describing (again, Super Metroid is probably the most famously non-linear """linear""" game of all time, and Zero Mission was made to facilitate sequence breaking), but that none of this excuses the fact that your more specific points you laid out are still wrong. You can say the responder criticized your trees and not your forest, as you just did, but blaming them for it when you is somewhat hypocritical considering you literally number them as to invite discussion around specific points. 

Part of the problem with your post and with this thread in general is that users keep flip flopping between inviting the 2D comparison and not inviting it. You seem to be arguing for a 3D game to do all this (hence point number 4), which makes sense given the current gaming climate and the high popularity of 3D games, and that's certainly a scenario where not everything on your list has been accomplished by a prior 3D game. However, the issue is that when we're talking about sales we can look to the past and confidently point out when 2D games have done literally everything you've asked (except maybe the vague point about setting, though Super essentially has two entire ghost ship segments in the game), and those games, relative to the rest of the franchise, still didn't garner exceptional sales. This makes your argument more indefensible - there are 2D Metroid games that have done all this, and yet despite that they never captured the sales of something like Prime or even Metroid 1, even in eras where 2D gaming was still the norm. This makes the criteria more flawed and also begs the question, why are you acting like only Metroid 1 did this? I understand that you fancy the NES, but it just comes off as a bit ignorant honestly, especially when you push it to the extent of acting like later Metroid games are more imitation less Metroid. On the topic of originality, Fusion and Prime might as well have a mission statement pinned to their game cases that say "We basically made something entirely original, but still based in the Metroid universe". Seriously, when you look up the history of 3D Metroid development, it's actually amazing how little the Prime games have in common with a standard Metroid entry in terms of original iconography, they really earn the subseries moniker. Hell, the only boss fight to reference an earlier game (aside from Ridley) was cut entirely from Prime 1. Fusion is a little bit more direct in it's connection to the original Metroid games given it's technically a Super sequel, but it's response to them is basically to make original creatures and game mechanics to parallel and even replace the dangers in the original games, not to copy them. 

All of this is to say, if you said something like, "Metroid could become a lot more popular with a 3D Metroid game that facilitates the appeal of the 2D games through the lens of Resident Evil meets Dark Souls meets Prime in third person", I don't think anyone would argue with you. I mean, people who don't like or haven't played Dark Souls and Resident Evil might be a bit confused, but it's actually a very rational analysis. Despite rarely getting brought up in discussions around the term (except Dark Souls 1 which is brought up more often), about half of the Soulsborne and Resident Evil series constitute as Metroidvanias - and they essentially dominate the sales of the genre, even being more popular than Metroid. And your arguments for how to make a good 3D Metroid game aren't even bad. What I don't get is when you act like Metroid 1's torch has never been passed, when it has many times (and it never got the sales to back that up, mind you). 

Perhaps I should have just said, "Metroid could become a lot more popular with a 3D Metroid game that facilitates the appeal of the 2D games through the lens of Resident Evil meets Dark Souls meets Prime in third person". I am providing the lists to add details to this sentiment.  I am not saying that zero of the things in my lists have ever been done before.  I am just saying listing what made the original game great and the developers need to be intentional about following that philosophy.

However, you also said, "To be fair, you did make an entire list of things Metroid should do, then got confronted with the fact that Metroid has done basically all those things. And instead of backing away you retreated to some vague idea of "none of the games capture Metroid 1's ideals", without actually realizing that not only did pretty much every 2D game aside from Metroid Fusion capture the exact ideal you are describing (again, Super Metroid is probably the most famously non-linear """linear""" game of all time, and Zero Mission was made to facilitate sequence breaking)"

This is not entirely accurate.  Let's look at what I actually did say:

Before I made my list I clearly said to get back to the underlying philosophy of Metroid 1.  That is the real point I am trying to make.  The list is not the real point.  The list is to give details to the real point.  I am not retreating to something vague.  I am telling you the main point.  And it isn't vague, because then I list details right afterward.  What I'm saying only becomes vague when you separate the list from the main point.

It's like if I told the waiter, "Make me a burrito.  I want beef, lettuce, cheese, beans, and sour cream in a tortilla."  Then he comes back with a soft taco. "Hey, this isn't what I asked for."  He says, "yes it is. It has beef, lettuce, cheese and sour cream in a tortilla.  It has the ingredients you asked for."  But the fact remains that not only is it missing beans, but more importantly I asked for a burrito and he gave me a damn taco.  The most important thing is at the very beginning I said to make a burrito.  When the waiter ignores that part, then I could end up with anything.

I am trying to describe the design philosophy of Metroid 1.  That is the most important thing.  If you miss that part then it's easy to miss one ingredient.  For example you mentioned Super Metroid.  Super Metroid is too easy.  It diluted what was great about Metroid 1.  I haven't beaten Super Metroid, because I get bored playing it.  I want to go back, but I can't.  It's so boring that I can't make myself.  It's true that it's very similar to Metroid 1 in a lot of ways, but when you take away the challenge the game stops being fun.  That one ingredient matters.

I know the internet repeats how great Super Metroid is, but you also have to realize that the game is most likely to be talked about by it's fans.  I am not a fan of Super Metroid.  I am not a fan of the Metroid series in general.  I am a former fan.  I really love the first game and don't care for the others, and I have tried several.  The sales numbers show that I am not the only former fan that preferred Metroid 1.  

Before BotW I would not have called myself a Zelda fan either.  I was a former Zelda fan.  Now I am a Zelda fan again.  BotW went back to it's roots in design philosophy and brought in former fans.  Also in trying to bring in former fans they found a whole bunch of new fans.  It may look like A Link to the Past is very similar to Zelda 1, but it's too easy and, although open world, it doesn't give freedom to the extreme degree that Zelda 1 or BotW do.  I do like ALttP, but I also feel it diluted what was great about Zelda 1.  If I listed everything about Zelda 1 I liked, then you might say that ALttP does the same things, but if you look closer then it really doesn't.

That is what I am saying about Metroid.  I am a former fan.  I am trying to say what I liked about the first game and why later games don't have it.  A couple of you are saying, "yes, they actually do."  But I insist, no they don't.  Later games do not capture what was great about the original.  They are similar in a lot of ways while still missing a key ingredient or two.

The original Metroid was the game that got me into gaming in the first place. The main difference I felt between it and Super Mario Bros. (which I got bored by fairly quickly at the time) was very much the comparative freedom to explore that Metroid offered. That wasn't the only thing though, and it's not my favorite entry in the franchise. Super Metroid and the original Metroid Prime are my all-time faves, in that order. Metroid...well at least Metroid games preceding 2010 anyway...is my favorite Nintendo franchise by a lot. It's been their one ongoing "mature" franchise and their only female-centric one. It's the one that is not typical Nintendo. I've never been that much into typical Nintendo, honestly. To that end, the absolute last thing I want the next Metroid game (...there ARE going to be more Metroid games, right Nintendo? Like before I die??...) to be is just another typical Nintendo type game.

I want the next Metroid game to be a Metroid game. Not a Legend of Zelda game. Not an Animal Crossing game. Not a cheery, happy-go-lucky, pointless, weightless game with cutified character designs (seriously, have we so quickly forgotten Federation Force?!), a Metroid game. One that actually retains the franchise's core identity. Not too many of those have been made of late, and none have in the AAA landscape in like...I'd say not since Metroid Prime 3 in 2007. I don't care if it sells. I want Metroid to go back to being Metroid again. I loathe the commercial attitude of the OP!!

I want the next Metroid game to be dark, desolate, substantive, driven by exploration more than action-combat, I want to star Samus Aran, and I don't want it to feature the Zero Suit or Samus's frilly new runway armor by Calvin Klein from Other M either or some "fatherly" commander telling me when it's okay to defend myself. No towns. No MMOness. No kawaii-fication. No selling out. Real Metroid. Sorry.