CSGO Hub and CSGO Servers. Maybe a 1v1 server? The hub could be a great way to get a 5 man queue easily if it gets popular enough. You could also do fun 1v1 events, or unboxings, or anything really. CSGO is a very versatile game when it comes to a community.
You could also do a zombie hub, with events where once a week maybe we get some people to play a classic WAW or BO3 map. It would also be a fun discussion area for people dedicated to zombies.
If you want a lot of people, GMOD is a really good idea for a hub/servers as well, that game is unbelievably popular and it's deathrun probably won't ever die. We could have really fun events take place, like maybe:
Obscurity event: An event where we play the game mode obscurity, which is a horror based gamemode where one person is a ghost like creature that has to kill off every player using special abilities, while the other players try to kill it.
Deathrun event: You could do this for cod4 too, but host events where we get a lot of players on for DR. With GMOD though, you can actually give rewards out for certain events. For example, log in during THIS event to earn THIS spray for a limited time only.
Zombies event: GMOD also has decent zombies, so we could literally just get a bunch of people on and find a server for zombies.