Here are some of my ideas for the Server, (Most of the ideas are Gmod related)
Jailbreak
Some kind of RPG Server - Pony RPG?
Prop Hunt
Mann vs. Machine (You can make it so people can play as Machine)
Deathrun
Engineer bots will be on difficult mode mabey?
Edit: I've played on a server with 16 engi bots once, it took forever for 10 people to kill all the engis once they have built. (Even hard for spies)