Underwater would be cool. I guess that would be the "outside" of the map, not sure if players can actually swim in water, never seen it on any map.
As for map layout I have a few ideas I've been rolling in my head...
1) Multi-level platforms, possibly moving platforms but something marines would also have access to (not just for aliens).
2) A map that includes much more height in it's design. For example, some rooms or tech points would be 1 or 2 "floors" above the base floor. The new Decent map did a little of this, and some of the older maps too, but I would like to see more of it.
3) Maze style map design where some junctions are made with 4 doors (N, S, E, W) which would force newer players to look at their map once and a while so they don't get lost! lol
4) Maps with "secret" rooms to take a time out when heavily damaged. Might make things interesting. Thinking of how they have hidden rooms in the lobby rooms...something like that.
5) Maps with a couple areas with more narrow hallways, something to use to change strategy a little so it would block larger units like Onos or Exo's.

I breifly looked into making maps with Unreal Engine for UT3 a long time back but didn't have the time. I might have more time for doing things like that now but I'd need to self teach. I am a fast learner and I have already done things with many languages:
Visual Studio .NET (Wrote apps for my company with it)
VBA (Expert in coding with this language, I can do pretty much anything you can dream of, but it's more geared to business use)
ASPECT (Old school language from Procomm Plus, discontinued but still used widely today in the tech world for scripting Serial Port stuff (i.e. good old COM1)
UNIX - know enough to get around and do basic stuff, manage lots of IP routers in our network with it.
SQL, Oracle, and general database crap. I call it crap because data is the bane of my existance! You know you are in data hell when you work laptop continually tells you that you are out of memory... :-(

-Derek