I voted maps, game types and mutators.
I don't care about models because they don't contribute to gameplay.
If the UI is outsourced it would be great, if they are the ones that need to spend time with it, then I'd rather have improvements like it was done on the patches. Some features are missing, but sometimes it's just a matter of removing what it's there. Redundant prompts, sub-menus, yet another layer of "advanced" screens, that lame ass background animation that makes the menu lag and take longer to load, etc...
I think they could reuse part of the UI from GoW, not sure. And then I'd focus on what Brad.G's tweaker doesn't provide.
A few maps, but there are plenty already and they tend to be used on servers, i.e. the work isn't being wasted. CTF-Searchlight and FacingWorlds must come on that pack again, I don't see them being used on servers.
That leaves gametypes, mutators and game features. As I posted before, DOM, BR, FreezeTag and RPG are already done by the community, so it's just a matter of giving a compensation to the modders and bundle their work. So that it isn't lost.
I'd like to see improvements for everyone, like new vehicles, a Necris weapon set, etc... For UT2004 players, the return of the LG, a nerfed shieldgun, improved Axon vehicles and lower gravity on vehicle gametypes. Freedom of movement is very important when healing/destroying nodes. I hope that you can read that "lower gravity" and "vehicle gametypes" are on the same sentence
For the competitive scene, what has been requested. Ping compensation, throwweapon, decent demo recording and UTV are very important to me. Some features from UT3Comp would be great I guess. And I'd be very happy to give up of some other things I'm talking about, just to enlarge that Ut3 community
And a different kind of DM map, with Necris weapons for instance. People must learn to adapt and move out of their confort zone! Some players consider the DarkWalker an interesting twist, it is on just 1 map out of 10
The priorities for all this depend on the manpower they use for the expansion, really.