I want to see some reviews first, but Just Cause looks like it could be a ton of fun and different than most games out there.
Biggest upcoming 360 game: Alan Wake | 18 May 2010
May 18th is a big day:
Lost Planet 2
Alan Wake
Both heavily anticipated titles. The former is a very fun shooter set outside in the cold wilderness with vehicles, grappling hooks, etc.
The latter could be a very original adventure exploring a new world and creatively using flashlights.
Some other options that won't sell a million copies:
Just Cause 2
3/23 release date
Game Room comes out on 3/24
It is an arcade room for your avatar (user) where you can add tons of classic arcade games (pacman, centipede, etc) to your room to play, maybe even with friends. I'm not sure if you can pre-order anything for that, most of that would be purchased via marketplace points, those 1600 pt cards you see in best buy and such. There are almost no deals on those (if you find one, it's like 5% off).