We should get a game type where you fight never ending waves of various campaign enemies with like 4 friends online that would be awesome.

Maybe not a simple firefight-mode, but something added to sp and mp would be nice. But not really possible. It would be hard work and it can be very complicated to add a complete new gamemode to the game.

halo reach adds all new content all the time it can be done they added online co-op after launch and have all new modes if crytek cares about its game more than money they will keep adding content

Halo Reach always came with co-op. It was specifically labeled on the box. They don't chance content either, they just change weapon types and rules to change the gaming experience.