It might be a new Star Fox game, though I think that would be a bit anticlimactic if ti were the case (nothing against the idea of such a game, but Star Fox stopped being a foundation franchise a long time ago).
It seems early for a new Zelda, but Majora's Mark did come out fairly shortly after Ocarina, so maybe it's that. Personally I think it might be Wii Sports II (or More Wii Sports or whatever it'd be called), though it wouldn't quite fit the mold of appealing to both hardcore and casual gamers (though you could make the argument Wii Sports does exactly that, it's not what people think of when they think of such a game). I wouldn't mind seeing them take a slight risk and making the big game be an entirely new franchise.
Another argument could be make for it being a big Pokemon game. Since Wii has online, Nintendo can finally create a similar experience to the handheld version, and make a console game that lives up to the handhelds (funny to think of a franchise being better on handheld, huh?). In fact, now that I think about it, I think this is it--this would be the ultimate killer ap for the Wii, bigger than Brawl or SMG, possibly.
In order of likelihood:
1. Pokemon Wii
2. More Wii Sports
3. Zelda Wii
4. Star Fox Wii
5. Donkey Kong Wii (which would be AWESOME)
6. New Franchise or Animal Crossing or Pikmin
I think Retro would be the perfect studio for Star Fox Wii, actually. I think they're a vastly underutilized resource for Nintendo that could make Star Fox be a true immersive, off rails experience that'd still work well. They've proven they can do shooter/adventures well, which is what such a game could be--an Arwing based shooter/adventure.
My consoles and the fates they suffered:
Atari 7800 (Sold), Intellivision (Thrown out), Gameboy (Lost), Super Nintendo (Stolen), Super Nintendo (2nd copy) (Thrown out by mother), Nintendo 64 (Still own), Super Nintendo (3rd copy) (Still own), Wii (Sold)
A more detailed history appears on my profile.
