While i've been a fan of RTSs since the days of the brotherhood of nod, it does seem to be much more difficult to find good ones these days.
In particular co-op RTSs seem to be non-existent and most that do support it seem like it was added at the last moment on a whim. If you're interested in a game that has more focus on the S part of RTS, and excellent co-op opportunities, i recommend
AI War: Fleet Command. It's an indy game written by a developer who actually cares about it's playerbase(No i'm not that developer, but I do play the game), and makes free DLC available almost every week with bug fixes, gameplay improvements, new units, etc. The gameplay is very asymmetrical. The enemy has already taken over the galaxy and is now distracted with other pursuits. The more planets you capture and the more structures you destroy the more annoyed the enemy becomes, sending larger and more powerful fleets against you. You can't go recklessly taking over every planet you encounter because the enemy would soon be mighty pissed and send everything it has against you.
It's not for everyone, however you should at least check it out if you're finding the RTS platform has been lacking as of late.