Ew, please no caves. The discovery of ghost merging and cave maps towards H3's end just about make me claustrophobic.
Funny you should bring this up, I'm actually working on one which I think will be playable and fun, if on the claustrophobic/close combat side. I've spent several days getting the geometry just right - I want the tunnels to have smooth floors (or smooth enough to run across without bumping) and enough overhead space of course, and for everything to fit together as seamlessly as possible so that you don't run around identifying which pieces were used where. I'm pretty satisfied with my efforts, but I still have some tweaking to do - mainly getting the lighting just right, and adding some visual landmarks for player orientation - then after that, weapon placements, spawns, gametypes, etc. I might be done with the rough draft this weekened. I think it will make a fun small game map - probably no more than 3 vs. 3, and maybe ideal for 1 vs. 1 or doubles.
If you want some "cave" maps then there are two in my sig: Ore and Caveside. They aren't really true cave maps, but they're cool.