Super Mario Galaxy was an awesome game and I will no doubt buy the new one, but I really don't want Nintendo to milk it. They had virtually nothing new to show...
Twilight Princess was basically Wii Zelda. Yeah, it's also on Gamecube, but it was marketted and sold primarily as a Wii game, if I'm not mistaken. Another Mario game would be sweet. Keep in mind the 64 and the Gamecube only had one main-series Mario game a piece, so getting a second one on Wii would actually be doing pretty well based on what we've seen the past couple generations. I'm hoping "Mario Galaxy 2" is a working title, though.
That may be how they marketed it, but it was in development for the Gamecube, and the two versions are identical save for the fact that the Wii version reverses everything to make the remote your sword hand, forcing you to waggle your way through swordplay (I hated this). The main benefit the Wii version had was the ability to aim your bow with the remote.