You should include "level cap increase" and "more appearance modifications" in your poll.
More outfit's would be my number one choice, maps/campaign missions as a second. More maps is nice but as long as they include the narrative style from the campaign with them.
eg. loading screen dialogue/opening cutscene and cutscenes between rounds and at the end of a level, thats one of the aspects i love about the game, your not just randomly doing a bunch of missions as theres some immersion included (and of course if people dont like that, the option to skip is always there ;p )
Another thing i'd like to see at some point would be a lobby system, would be great to make grouping with friends so much easier, fire teams are a start but not quite enough i feel.
I'd also add for the current bugs and problems to get ironed out, but im sure SD is working hard to fix those anyway, and dont need to be told again and again
New classes would have a high risk of destroying the game balance they worked so hard for.
New abilities might be interesting, but might have the same issue as above. Level cap should NOT be increased no matter what, I already feel that they have given out too many abilities per character. You can literally take every universal ability you like and every ability you like for a certain class and still have some left over.
And gotta agree with what Shadowcat said, more factions/classes/abilities while interesting would take alot of time to implement and would most likely mess balance up.
Plus we do have enough points for abilities as it is! More is always nice, but really not needed, in my opinion ^^ (though i doubt that would break the game as much as extra factions/classes etc would)