I'd like to see a lot of the stuff from MW2 Brought over into all FPS's (Challenges, Titles, Perks)
Better Anti-Cheat systems
File consistency/validation tools (required check of core files before game launch/server join - prevent hackers from launching any scripts once in game)
Bot Support
Variable Physics/Modes IE. (Hyper Death Match - UT/Quake Style Speed/Movement, Hardcore Team DM - CS MW2 style movement/gameplay, etc)
Loads of: Weapons, Unlockables, Maps
SDK/Map Building Tools