I'd prefer something from the older games, but wouldn't be surprised if they whipped up something from Super Princess Peach, like Rage (where Peach sets on fire and does double damage) or Calm (where she slowly restores health). Those are both cheap, but I never actually PLAYED SPP (I really want to get it, tho), so what do I know
But some ideas I like are the Psycho Bomb idea and the Peach Bomb idea. They match her personality pretty well, and they'd both look really cool in-game.
I could see her doing a few others, tho, like a Toxic Kiss of some sort that dazes the nearest opponent, a Heart Shield that protects her from attacks for a while, and some stuff from MUGEN like the Warp Whistle (giant tornado that sweeps the screen) or the Orgel (puts everyone to sleep, a la Mario 3)
...this has inspired me to post more Super Smashes. Be back later
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)