While I'm still utterly bitter about this...game; I say we look to the future instead of crying over the past. So let's see a little life in here!
We got a fancy new home console, DYING to have games on it, and we have a game series dying to be used to its fullest potential. What would you like to see in a Paper Mario U game?
Aside from obvious stuff, like a return to the old turn-based system with items and badges and leveling up and partners, I wouldn't mind them bringing back some of Sticker Star's sticker elements. Replace badges and items with stickers. Yeah, you heard me right. Think about how a badge-stickers system would work. You "apply" them to yourself for their effects and can "peal" them off later and replace them with others. It would be one of those cutesie details that shows off the PAPER of the Paper Mario series, but doesn't beat you over the head with it, as 90% of Sticker Star's jokes do. You can still solve world puzzles with your sticker items, like using one of those good ol' Courage Shells ( which is just a regular Koopa shell, but you use it to boost your defense) on a Koopa without a shell, or stuff like that. A simple touch, but by no means the absolute focus of the game (blahblahblah referring to Sticker Star again).
And for the love of all that is good, do not use that boring, overused, trite Mario Bros 3 overworld. It is a relic - a darned RELIC from an older time - a time where games simply did not have the space to develop hub worlds. Galaxy 2 didn't need it. And although you can argue that 3D land was homage to MB3, and that all FOUR of the NSMB games are a homage to MB1-3, it has no place in Paper Mario. JUST NOOOOOOOOOOOOOOO. I beg, nay, I command for a proper world in the next Paper Mario game. *pants*
...Soooo yeah. Any other ideas in mind?