1. Keep it cheap $250 or $300 (no more than that)
2. HD graphics - at least 720p, preferably 1080p
3. Good online that can compete with Xbox Live and PSN
4. Some cool new Nintendo innovation to keep it fresh - it can't just be better motion controls IMO, they have to do more than just a Wii 1.5 to get people hyped for it.
5. Keep the focus on fun and playing with friends, don't make it hardcore