So, if both casual and hardcore will be pleased...
I'm guessing maybe:
A new Punch-Out
A new 1080 Snowboarding
Animal Crossing (guaranteed @ E3)
Kirby (guaranteed @ E3)
Disaster (guaranteed @ E3)
Kid Icarus
One or two new IP's.
The first two will get hardcore gamers salivating, and both will appeal to the pick-up-and-play aspect of the Wii for casual gamers.