-More worlds. Maybe make one a callback to CCW and give it the 4 seasons treatment with a different layout
-Unlockable clothes or accessories. For extra fun, make the player earn it by trading a super rare item. Alternatively, if the game's worlds are heavily themed (western, pirate, etc.), then give them a fitting costume for the duration of that world (cowboy hat).
As for things that may or may not be unrealistic (I really don't know):
-I would love a "64-bit" mode with awful draw distance, low-res textures and the like.
-I would love online functionality of some sort. Whether it's through online coop, or something equally ambitious, but fresh. Maybe a Devil Bottles online mode where you hop into someone else's quest and ruin their day if they allow the setting.
After that, I'd love to say things like "better animations!", "better textures!" or "more varied mini-games and challenges!" but I have to wonder at what point are we adding stretch goals that would have ended up in the project anyway