It's difficult for me to express how much I disagree with this.
The absolute worst decision was to canonize the manga.
Not 100% certain on this, but I'm pretty sure Nintendo has never made the manga canon. Sources to this if they have would be nice.
If any franchise deserves a reboot, its Metroid. Just get some good writers and start over. Do it right this time. I'm all for a Metroid game where Samus is better characterized, but it should be more like System Shock 2, Bioshock, Half-Life 2
...Gordon Freeman and Jack have even less characterization than Samus. Even throwing Other M out the window, Samus, at the very least, has her monologues and discussion with the computer from Fusion, the way she reacts to certain situations shown in cutscene (particularly bounty hunter deaths in Corruption), and quite a bit of backstory from Chozo Lore.
If there are NPCs in the game, they should be like characters in a Souls game or, again, Bloodborne. There to lend credence to the overall dreary tone that Metroid should hold above all else. They build the world, and that's it. No witty banter. Just dreary, even sarcastic, dialog
With the exception of Other M, most of the NPCs in Metroid games exist to build the world. The hunters in Corruption, for instance, serve as a way to display the corruptive power of Phazon, by showing how easily they dispatch tough enemies and then how even they fall easily. Space Pirates (not a traditional NPC but still) serve a similar role in Prime 2 with the Ing and Dark Samus. Someone like Admiral Dane or the Aurora Units show up for little reason other than to advance the plot and to give us an idea of how the Federation works. There are very few characters who show up to simply have witty banter (*cough* Anthony *cough).
Sarcastic dialogue would be all right, I suppose, but I'm not up for consistent dreary dialogue, even on the point of Dark Souls. Much of the talking from NPCs in Dark Souls is meant to convey how utterly screwed the world is, and quite honestly I'd argue it gets pretty boring and samey after a while. I don't think I'd want that in a Metroid game, especially since Metroid isn't about a sense of utter hopelessness like Dark Souls.
Everything should be voice acted and instead of the outdated Prime way of scanning everything to read endless flavor text, there should be the equivalent of Audio Logs throughout the environment. Most of the information your receive should be visual, though. Once you see a federation trooper stabbed through the heart by something, you don't need a piece of scanned info reiterating that yes, this is a federation trooper and yes, it was indeed pierced by something. Samus can hack computer for world building flavor text, but that should be a rare occurrence and not nearly come up as often as they did in the Prime series. They are much better ways to build a world, as mentioned above.
Absolutely not. At the very least, give the player the option of reading the text instead of listening to it. Reading allows the player to cover the information at their own pace (as opposed to possibly missing a word from an audio text and having to re-read it), while simultaneously making the process quicker. For instance, if you're a quick reader, you can move through a text file in maybe a couple seconds and move on to something else. If you force this into audio only, you have to wait until someone finishes talking if you want to get all the info before moving to something else. Text allows the player to gain information at their own pace.
In a Metroid reboot, Mother Brain should be a Shodan-like (even Glados-esque, minus the humor) villain. Cold, calculated, and intelligent. There should be genuine fear built up to the final showdown with Mother Brain, and that should be built throughout the events of the game. She is, far and away, the most interesting and underutilized villain in the Metroid franchise. Ridley is nothing more than a pirate captain, and shouldn't be treated as anything more than any other monster.
...Ridley is actually the leader of the Space Pirates. I believe (not 100% sure on this, but if the manga/Other M is enough to gather impressions off of), Mother Brain used telepathy/mind control to take command of the Pirate forces during the two Zebes incidents, sort of in a similar way that Dark Samus used Phazon to control them during Corruption.
Metroids should be scary again, because they aren't anymore. They are a far-cry from what they were before, and that's because we never fight anything other than their larval states. In Alien, face huggers are just the babies. Imagine if every Alien movie after only had face huggers. We should be fighting Zeta Metroids by the middle of the game, and they should be the equivalent of Big Daddy fights in Bioshock. Extremely powerful, fast, and aggressive and extremely difficult to defeat without proper planning before hand. Most importantly, they should be fucking scary. You should see one and shit your pants. Hell, you should even see an infant Metroid and still be scared a little by the end.
Metroid isn't a horror game, dude. It never has been. It's not supposed to be. There are tense moments, but the series has never been aimed at making its players terrified. Metroids themselves were never particularly scary.
What would you even do to make them scary? Put giant teeth on them and have them jumpscare you or something?
One final mostly irrelevant point of canon; Metroids can't actually evolve without being on the atmosphere of SR-388 (or an atmosphere manufactured to mimic it). That's why they're only in larval form in every game besides 2 and the ones that take place aboard space stations with atmosphere replicators.
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Above all else, though, none of this particularly convinces me that Metroid needs a reboot. I do like some of the ideas for gameplay changes; I'd personally like to see Nintendo take a stab at another 3D 3rd person Metroid, but what purpose does a reboot serve? There's still plenty of story to tell in Metroid (preferably FINALLY getting around to resolving that cliffhanger at the end of Fusion), and I'd much rather have Nintendo try to come up with some original villains rather than simply pressing the restart button and bringing back updated versions of the same old bad guys we've been blowing up over the past 28 years or so.