Some additions I've been waiting for have been a Metroid and Lightning Bolt to show up as items. I know I've mentioned both before, but that was probably a year ago, or more. I like that they've already put in Nintendogs, and I look forward to what a Chibi-Robot would do.
If the Metroid were there, I would like it to do something like 20-30 damage before returning to you, to refill that much health back. A lightning bolt would basically be a Super Mushroom, but with a different perspective. Everybody else gets small, and you stay, instead of everybody staying, and you getting big. Alternitavely, it could come down in bolts, like Pika's, and shrink whoever gets hit. Either way could work, really, although I think I prefer the second, only because it offers something different/unique.