How about a sickbay? We don't currently have a pre-made one.
Doesn't have to be that, but if it were me, I would think along the lines of something we can't get on any Cryptic maps. For instance there aren't any astrometrics labs and I needed one for "Somewhere in Time" so I made my own.
Possible ideas, big and small:
-Ship's observation lounge. Cryptic has made one, but its not the foundry.
-A Starfleet Academy interior, maybe with classrooms and a bridge simulator! Since we can't really use the exterior for anything.
-Average Klingon's house
-Alien starship interior. Something far more alien than the generic ones we have now.
-The Lower Decks. i.e., the bowls of the player's starship.
-Astrometrics/Stellar Cartography. I've already got an entry for this one
-Federation Council Chamber. Big place for diplomatic meetings.
Those are largely interiors, or interiors that would be build on ground maps. But if you're looking for more of a ground exterior building, how about a crashed starship. That would be crazy hard to make it look good. Or maybe an orbital lift, like the one from that voyager episode!