It likely will be another Colloseum/Revolution for the Wii. It was released a couple months after Diamond/Pearl so that's my guess. Battle Revolution was fun, but short, so a larger setup of colloseums could be set up. Plus, Triple Battles in 3-D equals win.
It probably could be another Mystery Dungeon game, which I find strangely fun. If so, I'm hoping it doesn't have those atrocious Pokemon Ranch freaks like Pokemon Rumble and that japanese Mystery Dungeon game for Wiiware did.