My friend came up witht the idea of a base war type of thing, you make a set time to build, then you can purchase weps and props and even npc to fight with u, you can also add ppl to your side, the gametype would be a bit like flood with set times and winning bonus but water never rises and your tools are like zs, you pick a prop up and nail it, or if ppl dont like that ou could use phys gun, each base must have a "core" of some sort placed in the base and when you are fighting, you destroy props untill you get to the core and when yo blow it up, that persons team loses. Some of these things might not be doable, but i just wanted to post because we had an idea, feel free to comment and give new ideas.
here is a gamemode we can use or edit or even use as a template, you decide:
http://www.garrysmod.org/downloads/?a=view&id=79542