RTS - an absolute must for the wii
dual wielded wii motes - considering the sales of wiiplay you know everyone has a second controller.
myst style exploration puzzle game
World of warcraft - okay, not the full mmo part of it but i really liked playing the game. a 3rd person action based game would be awesome. use the nun chuck for the asdw movement controls - an action bar across the bottom of the screen for your 20 or so spells that you use the pointer to click your spell. highly complex and deep combat without the highly complex controls. hell, the wii has online - make it an mmo.