Adding new gamemodes, maps, weapons and vehicles to Crysis 2 multiplayer and the sandbox would be a phenomenal idea. Let's customize our own gamemodes! In Halo 2 they had FFA, Team Deathmatch, CTF, Territories/Domination, King of the Hill, Assault/S&D, and some other stuff.
Shower us with gamemodes and new maps and weapons and vehicles and features and sandbox editor and dedicated servers, lol
^happy community

Another idea would be a scenario for each map; activate this, then open this and hack this, then go to this helicopter: something to put the entire team behind. Make it so you can tag locations with the visor or draw points on the minimap to form a path (careful though the little kids will spam these)
Add more killstreaks to new maps! CELL Helicopter, A-10 ground assault, air transport (VTOL)
Make it flexible! Assault scope for SCARAB and silencer for SCAR
Add DMM <- consumer satisfaction 99%
Procedurally destructible red striped concrete blocks


EDIT: New idea: Community map of the month or something