General advice:1)Make it different, but simple.
2)Don't include stupid gimmicks.
3)Make it dark enough.
4)Think about the balance in big and small servers.
5)Make it fun!
Ideas:1)Futuristic neo-gothic style mansion
2)Make an underground vault, with something weird inside.. not money
3)Big broken
fishtank, with dead fish inside (
some kind of prehistoric fish)

4)Security center, with
lots of screens (textured, not funcional).
5)Highly protected outside yard, something industrial. Somthing like
this but a bit more refined.
The whole idea is to make a tunnel from the yard to the vault (aliend dug it i guess..).
-The alien base will be in the big room in the middle of the tunnel.
-The human base will be somewhere in the first floor. Near the fish tank
The vault will have 2 entrances, the
main door and the dug tunnel.
Here's a l33t drawing, be amazed

You will call it
Basilisco's dream map LOL
[But seriously, i just spent like 45 minutes doing this.. hope you use some of the ideas

]