Well, if other games are anything to judge by, the DLC won't be able to change the AI, or add in a team command option.
If weapons are going to be tweaked, they will probably just tweak the stats with a normal game update, and the same for the new game modes- like they did with the big teams.
To add more to the story, it'd have to be quite a substantial DLC pack, which I think they'd have to have spent more time on in order to do, and with a big pack like that, they'd probably add in more clothing with that style of DLC.
I can't see them adding more abilities, unless they increased the rank cap, which would probably be already on the disk if they were planning on doing it any time soon, and they'd just use a download to unlock that data and enable it.
The only thing I can really see this free DLC including is more maps & challenges, especially because its being released in such a short space of time. A big part of the DLC will probably be a title update, which will fix a few bugs and glitches, like the 'light body with minigun' problem.