I've had this idea for a while:
Peach and Daisy are playable as either tag-characters (DKC 1-3 style; 1 player), or co-op characters (NSMBW/DKCR style, 2 players). Perry also returns, assisting with his own abilities, too.
The girls start off with just the usual platforming moves, as their magical abilities (from the spinoffs and RPGs), used to solve puzzles, fight, etc., have been stolen by the Koopa Troop (who also made off with the Mario Bros. and Yoshi and their abilities, too!), and they regain them by beating (mid)bosses (Mega Man style).
The gameplay is similar to the Yoshi's Island games, DKC games, and SPP.
I'd also like to add that the game would be as hard as Yoshi's Island and the DKC games, with more complex puzzles and tough platforming.
SPP, while fun, was too easy for my tastes, and the Vibes really didn't help Peach's image, either. Sure, they're a pun on how Peach can control her emotions (She's the only one in the game who can literally do it without the Vibe Scepter!), but people obviously took things the wrong way…