This is a list of my PERSONAL opinions that I don't expect everyone to agree with and that might be somewhat controversial.
Multiplayer:
-Ranked servers should have a maximum of 12 players. This is the amount of players the maps and game modes were designed for, since its the most 2 out of 3 of the platforms can handle. Adding players above this number decreases skill and makes the game spammier-- especially on smaller maps like "Terminal", which can only really be experienced as they are meant to be played at smaller player counts. Moreover, high player counts cause spawing issues which have no place in "ranked". The idiot masses whining for 32 player servers without regard for map size can keep playing as they are-- in unranked. At the very least, make 16 players maximum for PC, as 18 is just silliness.
-You need to increase the recoil of just about every weapon in the game for the PC version. Aiming with a mouse is just SO much easier than a gamepad and the game lacks some of the challenge it should have. Considering Crysis 2 does SO much to encourage a very high skill cap, its sad that the essential gun-play element of the game is so easy.
Singleplayer:
-You have GOT to add a New Game Plus mode. It's pretty much there, it just doesn't have the title and it doesnt give any special rewards. This is a really sad missed opportunity to encourage players to play the game again with the knowledge that they'll benefit from suit upgrades.
DLC/future updates:
-You really need to add a firefight mode a la Halo: Reach. The enemies and AI in this game ARE TOO FUN to go to waste and it provides a gateway for casual players to get into the game. Perhaps the twist in the Crysis 2 version can be that you unlock dog tags from killing CELL/marines, which can be used to call in kill streaks, and you get nano catalyst from aliens, which can be used to upgrade your suit in a more detailed way than singleplayer.
-"Crash" is by far the best game mode in Crysis 2. Usually KOTH-type games are style, but the nanosuit makes it incredibly fun in Cry2. Experimenting with other modes based on Crash, such as having multiple crash sites at all times, like Domination in the Call of Duty franchise, or perhaps add other gimmicks. For example, a primary and secondary crash site simultaneously, with the secondary crash site being smaller, and only benefiting from having one or two players on it before it becomes maxed out.
-A game type that has CELL vs Marines while hostile Aliens intermittently crash land and attack. Again, a game mode designed to emphasize the great universe and AI in the game.