-If you know how to mix sound: Add sounds of gunfire to other 'sealed' rooms. Rooms you cant get to, add a quiet drum of turretfire and machinegun/shotun fire, and make it sound like theres some distant combat by the doors. Keep it short though .wav filesize can eat at a download time.
-Add some pointless dead-end rooms here and there, but not too many. A office or bathroom here or there can make your map seeming like its all hallways/hive rooms/marine bases.
I had some more, but those should be noticeable in a map
