Even though they're all slated as "remasters", I still hope the 3D Mario remasters if/when they happen add some extra things. For 64 I hope they at least keep playable Luigi and Wario from DS. I can understand not adding Yoshi as he was the 100% reward in the OG and they want that nostalgic factor to be there.
For Sunshine, I'd like an actual proper Shadow Mario boss and maybe some more cutscenes or context regarding exactly how he spread entire lakes of goop around. Maybe we get like a flashback from Piantas who witnessed him doing things, becauae throughout Sunshine it's only ever scarcely implied he caused a ton of **** but we never see it happen, and when we actually face off with him he's practically a joke who flees the entire time. Shadow Mario is cool and he clearly has the ability to fly and teleport among other things, so just more of an expansion on that form would be cool.
For Galaxy... Oh boy. With the graphical updates maybe make it clearer that these are galaxies and planets and such. Take inspiration from Kirby's beautiful cosmic backdrops so that way we can actually believe we're traversing through a galaxy. Maybe if these are all tied together we can find a way to spice up BJ's boss fights via adding more projectiles to dodge in his airship fight via him spitting flames as well as pulling out the brush and flinging some graffiti at you. Maybe "Cosmic Mario" is shown to be a form BJ can become akin to Shadow Mario and that explains why losing to Cosmic Mario results in a death? Because Bowser Jr. is hindering your ability to progress and save the universe by nabbing a Power Star? Maybe more clarification on exactly who Rosalina is (this one is probably the most pipe dream-iest of all which is actually sad). Maybe since Captain Toad debuted here we could have like side-modes or missions involving a Galaxy-themed Captain Toad adventure.
As for 3D World... More clarification on why the Sprixie's are kidnapped instead of Peach or something because they're supposed to be relevant characters to the story but they don't even have dialogue to even give then character like the more forgettable 8-Bit character back in the day did. We don't even HAVE a story in thus game at all beyond "we gotta save them" when even the instruction booklets for the older games at least tried giving us a narrative. Beyond this, I half-expect Toadette to be shoehorned in somehow, even though chronologically she's helping Captain Toad during this game. Oh, yeah, they'll probably make her a selectable character for the Captain Toad stuff. As for the base game, If they did add a character, I could see Nabbit, Wario, Yoshi, or Daisy (most unlikely) being the sixth character.
And that's all I can think of rn