I'm not saying everybody can make a selling FPS, but hey, if it's on the 360, it will likely make a good return, add DLC sales and for a lot less work and time you can have a winner. If not, copy things like Army of Two, Kane and Lynch perhaps? Co-op, DLC, anything with guns and multiplayer goes.
EDIT: A quick look at some shooters using (meh) Metacritic's list and their (shitty) Metascore, but just for reference this works.
Meta score sales (vgc data)
Army of Two 72 2.5m*
Wolfenstein 72 700K*
Turok 69 1.25m*
Kane & Lynch DM 65 1.38m*
Dark Sector 71 800K*
Mercenaries 2:WiF 72 1.71m*
*combined PS3/360 sales
Just a few random examples. I know I could do a better list of below 75 meta that sold 1 million, but this illustrates the point pretty well. Go shooter, go multiplat. Score. That's the reality of this gen.
