New maps is the obvious one. It wouldn't be the biggest addition, but a bunch of new maps would be nice (especially some ones that break up the rather repetitive UFO mission maps right now, since apparently right now Earth consists of cities/suburbs, and great sprawling deciduous forests everywhere else). On the top of my wishlist would be a way to let modders make and introduce their own custom maps, but I'm not holding my breath on that one.
There's also room for a fair amount of tweaking and adjusting in the post-laser portion of the game. Note that introducing new equipment for the humans isn't the only way to add DLC. Introduce new aliens to add challenge. If done right, you could also mix in a new weapon or two that provide alternate but not clearly superior options to the existing ones (much like the current shotgun/rifle choice for Assaults).
With a lot of thought, it might be possible to introduce a fifth class of soldier into the game without unbalancing it. The risk would be either making a class that just duplicates a role already filled by one of the current four or making a class that is just too strong/weak compared to the existing ones... but I'm not prepared to say it's an unavoidable problem. All that said, I suspect if they have an idea for a fifth class of soldier they're going to sit on it until they can make it one of the big carrots for people to buy their first real expansion pack whenever that comes out.
There's room for significant tweaking of the psionics aspect of the game, on both the alien and the human part. A "Psionics Expanded" DLC of some sort could be fairly fun to play with if done well.
The way they seemed to be heading with the first DLC is in the form of relatively highly scripted story missions. That's not really what I'm interested in, but I can certainly see how some other players might enjoy that, and it's relatively easy to preserve overall balance while introducing those.