Yeah the cap-enemy-playing mechanic was shocking and awesome. I definitely think they need to revisit this one more time with a Odyssey direct sequel. Expanding and improving everything on an already fantastic game like they did from Galaxy to Galaxy 2.

Wouldn't mind seeing Odyssey 2 on Switch and then Galaxy 3 on Switch 2, then be done with those ideas and move onto something completely fresh.

Or hell, how about combine the two for the next 3D Mario: Super Mario Galactic Odyssey! Mario with Cappy in a Galaxy setting. And have a mix of the small worlds from Galaxy along with some a little bit larger but still Galaxy-style worlds so you can have some medium-sized environments for more in depth platforming and scenarios for cappy usage. Maybe like each level has a couple larger worlds as the centerpiece (still tiny compared to sandbox Mario levels) and a bunch of satellite tiny worlds.

Something that..
... is not a 2d side-scroller
... is not similar to Super Mario Odyssey (didnt like it and it was the first time that I regretted my purchase)
... what goes beyond what can be expected in a game for everyone/kids in terms of puzzle/problem solving difficulty

If you didn't sell Mario Odyssey, give it another shot.  Sometimes games need a few shots at it.  My brother started and stopped playing BOTW twice not really liking it before trying a 3rd time and loving it.  I've done the same on quite a few games like Skyrim, Final Fantasy X, Grand Theft Auto 5.  It really is the perfect 3d Mario game and quite addicting once it hits its groove.  If not that's cool, not every game is for everyone.  I personally can't get into Halo, Call of Duty, most free to play games, and many other popular games that just don't click with me.

Super Mario RPG 2. Bring back the cast, bring back the Smithy Gang with a new leader, bring back Geno and Mallow and Bowser and Peach, add Toad and Luigi and Yoshi and 2 more new characters for a proper RPG style cast (with the ability to swap characters out like in FFX).

That's all

Oh, and Super Mario Galaxy 2. Another GOOD yoshi's Island, and another GOOD 2D Platformer. Mario has only had 3 great 2D platformers (3, World, and NSMB U...yeah I know I'm weird for thinking that...it's the world maps)

Most people love Odyssey, including me, but yeah for sure not every popular game is loved by everyone. I for one find GTA boring, couldn't get into Minecraft and Fortnite. People who don't like Odyssey seem to just not like open level 3D platformers. They want platformers that are more like linear levels like the 2D variety. That's the only complaint I've ever seen of Odyssey. They tend to like 3D World and Galaxy but not SM64, Sunshine, and Odyssey.

Oh my god, weird for thinking that is right haha. NSMBU is I'd say the worst 2D Mario I've ever played by a long shot, in that it is pretty mediocre, while all other 2D Mario's are pretty fantastic. NSMBUD is a lot of fun multiplayer, but I had to force myself a bit to keep playing through it one player to beat the game.

That's just it, though...it had so much more to offer than any other 2D Mario outside of Super Mario World or Super Mario 3. It had the most unique and interesting level design maybe of any 2D Mario, it had the most secrets, it had dozens of alternate exists and hidden routes, it had plenty of ways to enjoy the game, there was actual interaction on the world map, etc. 

It was by far the most innovative, fun, and creative 2D Mario since Super Mario World. (I don't count Yoshi's Island, that's its own thing and it's wonderful). I used to think it was mediocre as well, but after learning a bit about level design through Design Doc and GameMakers Toolkit, I came to realize just how great the game's level design really is. I dunno, that's a hill I'm willing to die on. NSMB U and NSMB UD are some of the best 2D Mario has ever been and I was happy to pay full price to get them on Switch even after buying the game and expansion on WiiU. 

Die away my friend haha. It just felt totally uninspiring to me. Also the controls are too loose or sloppy or something. After playing it I played some NES Mario on Switch and was immediately like wow this feels wayyy better. It is hard for me to pinpoint what made it feel mediocre beyond it just feeling uninspired and loose controls, but yeah I had to force myself to keep playing to the end which has definitely never been the case for any other 2D Mario game.

We need Nintendo to start a new series of 2D Mario on the Switch. The New series was great for bringing Mario back to 2D but its time has passed.

Oh, I'll die on this hill, but I get it. It seems I am in the tiny, tiny minority of people who even like NSMBU, let alone think it's the best 2D Mario game since 1990

If they want to make a SM Odyssey 2 I hope they make at least twice as many kingdoms. Odyssey must be a masterpiece and I gave it a 10/10, but it's criminally short. Even if you are a completionist and don't get bored to explore the same kingdoms again and again you still can finish everything is like 60 hours. Indeed after only about 30 I was done with the game

You realize that 60 hours is VERY LONG for a platformer right?

Sure if you just do what you need to do to get to the end you can probably beat the game in 15 or 20 hours but that's because it doesn't require you to get a ton of moons to get to the end. I think it took me like 26 or 28 hours to beat it because I did a decent amount more than just what was required. Then I kept playing the game another 30 hours to get more moons. Still have a decent number of the harder moons to get. That is a very long game for the genre. Calling it criminally short is like calling Breath of the Wild criminally short and demanding BotW2 be at least 200 hours haha.