I'd love a proper Peach game that makes her more like the Peach from the comics and less like the pms'ing mess she was in that DS title.
When I think of the top tier of Mario characters (Mario, Luigi, Peach, Bowser, Toad), she really seems the least developed in regards to personality of the group spare Mario himself, but I've largely given that a pass as Mario is meant to be a stand-in avatar of sorts for the player him/herself (much like Link).
Luigi: Reluctant, frightened, overshadowed hero
Toad: Always in a panic, typical doting servant
Bowser: Quite funny in his mischievous antics, his motives are generally very petty
Peach: Always has the look of a stereotypical air-headed blonde, acting completely helpless, and exists only to be a damsel.
Now, when you consider Luigi, Toad, and Wario have all received quality, unique games, I'd love to see one for Peach. They really need to allow her a personality more akin to that in the comics and the competitive games (like Tennis/smash etc), though, and it'd be neat if it was of a unique style. Perhaps some sort of castle defender/pikmin hybrid where she directs toadlings and such?
It'd be fun to see something new at least. Something about seeing her get grabbed by Bowser in the recent video and shout the recycled (they didn't even record a new take) "MARIO!" made me realize that I've just gotten completely sick of her as she currently exists.
Also, I wouldn't mind seeing a Zelda title with Zelda as the hero either, though I'd prefer it not replace the next Link-based Zelda game.