thats a good question.
i found the online pretty tight, which is the only thing i actually played, so I really don't know what I want.

oh I do... customizable characters

make the single campaign longer!! loved the game but that needs improvement...

A little longer story mode than the first, and ALOT more guns, maybe even some prototyppe weapons like the XM8, FN2000, and the Kriss

EDIT: Also new map packs every 2 months

Co-op local multiplayer split screen

This is probably a given seeing as how it was in COD:WAW

I don't have huge demands. I just want Capture the Flag, War and maybe some new objective-based multplayer modes to make it into MW2. The former two are my favorite modes to play in WaW and I'd love to be able to have them in the modern setting also.

Maybe I would take better rewards for going prestige. I haven't done it in either COD, and that's because I've never seen the incentive to.

Oooo...one more thing. It would be nice if your 3, 5 and 7 kill streak abilities were chooseable, like you could have one of two choices to add another layer of strategy in there.

Zombie terrorists.

Let me add to that - on the Wii version ;)

Customisable characters for online.
More camoflage options for weapons + Custom camoflage
more maps. 32 players online. Dedicated servers. Bots for pri matches. A cover technique like killzone 2. More weapons. Zombie Terrorists/Russians.
Gimme this small amount and ill be happy

warfare inside of Tanks and Apache's. MP7s, Seals, etc.