Here are some of the games I have heard of:
1. Killer Instinct 3:
Rare has been working on 2 secret games for quite some time, I would assume that this is one. KI 2 was probably one of my favorite fighting games as a kid so I would be looking forward to this.
http://www.xbox360fanboy.com/2006/12/29/rumor-killer-instinct-3-for-360-in-2007/
http://features.teamxbox.com/xbox/1464/Most-Wanted-Unannounced-Xbox-360-Games/p2/
2. Mech Assault 3:
Didnt like the first two but none the less this is to be in development.
http://features.teamxbox.com/xbox/1464/Most-Wanted-Unannounced-Xbox-360-Games/p8/
3. Jade Empire 2:
The first one was great I t