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  

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