Games : Looking for action co-op game

I have a feeling this may not be the right spot, but I'm sure the good patrons at StackExchange will let me know where it should go ;).

I'm looking for a good co-op game, preferably some sort of FPS RPG. I play with my girlfriend, so I need to be able to play with just two people (unfortunately this disqualifies F.E.A.R. Online).

We have played many games like Don't Starve, Left 4 Dead 2, Borderlands, etc. We're looking for something that's action packed like L4D2, but has a bit more character progression.

I'm fine with giving an MMOFPS RPG a try. The game doesn't specifically have to be just two players, but we should be able to play alone if we want, or more if we want.

I've already exhausted the lists at websites like Co-Optimus. So, any suggestions?