How about a room, enclosed, with lots of boxes, maybe a single raised bit?
If you can't think of anything, try to imagine a basic area that you'd like to play on as a map, then base your ideas from that. Eg a car park would be fairly mad. You could then add symmetry etc. You'd have to be careful with that sort of map- one team could end up spawning on all the best 'higher ground'.
But really, just imagine some of the best maps you've played... base it on them but change it so they're more... you.