There are three different things that we have learned about the Wii that make me think of how they could be used in the upcoming AC:Wii
1. Caricatures
I think the custom face making revealed in Wii sports will also be used in creating your Animal Crossing character. This is going to give more variation in appearance than the current haircuts, hats and accessories. I think this could be an indication of the graphical style AC:Wii could take as well.
2. Integration
Speaking of Wii sports, what if that title could be integrated into AC:Wii? You could play tennis with Nook or fly a plane over your village. The Opera web browser could work in AC as well, visit a site in game to view news from abroad, maybe have an online auction house?
3. 24 hour connect
Nintendo has given us a taste of free downloabable content with Wild World, I hope we get much more on the Wii. I assume the visit your village after-hours will be optional, and visitors probably won't be able to affect trees or flowers or anything. Village to village mail should be possible now too.