I've been enjoying the coop in Saints row 3 very much this passed week. Maybe give that a go if you want some coop action. The entire single player campaign and all the side missions all work in coop.
The SP/Coop is seamless too, so you can take your save to either. Its fun game but i think the story in the second game was better and had better and more side missions.
During the last few days of the steam holiday sales I also picked up Ridd1ck Assault on dark Athena.
That too might be your cup of tea if you have not already played it.
X3 series is very nice and a huge time sink but isn't for everyone.
It's a space sim with a big universe where you fight or trade. So it's not fps.
What I think is really cool about it is just how expansive it is and becomes. You usually start out with a small class fighter ship, do odd missions, engage in some trading to make more credits, then buy bigger ships and start to automate them to do your trade runs etc to the point where you eventually can build your own factories and space stations and at the same time expand your fleet, space ships ranges from very small scout classes all the way up to huge carriers and battle cruisers all of which you can own, pilot or assign as wingmen. Each game has a single player story campaign but you'll still do all of these things as you progress through it. Or it can be played without story in various starting rolls like trader, fighter or pirate. It's a huge time sink but I love this series and the freedom you have there.
The learning curve is also much more steep then what one is used to in games these days I'd say.
Also as others here have mentioned, Mass Effect series. IMO you won't get much better sci-fi games then those. The back story and attention to detail in the tech is amazing I think and much of it "makes sense" and doesn't sound too implausible giving the series a sound foundation which to build it's strong story upon.