Yeh. Multiplayer is where it's at, what can they add you ask ? Alot.
More maps, for a start - more maps = more replayability. It's a simple equation. They can also offer better physics, better graphics, and more weapons (When do we get a game with the british standard assualt rifle !?) They could also offer more useful perks, and get rid of the crap ones (Martydrom anyone?)
They are adding vehicles to the game too, hopefully they make them optional in the game lobby, on or off. 99percent of servers will have them off, so that won't be a problem. But then - if they are well implemented, i'll remain open minded.
Also. Large maps ! my god if they don't make a very big map i'll scream, what happened to the fun of long range sniping ? Was impossible in cod4, the maps where to 'close quaters' and not enough large environments.
I would also like a map based in a abandoned city or town much like Chernobyl with many of the buildings open, i think that would provide some interesting gameplay.
Also, they should put in a element much like counter strikes hostages game mode. Where you capture the hostages, and one team defends and one attacks. This would ensure real tactical teamplay.