Yep, there have been a lot of threads about co-op. Not only that but as RockerJesse said, I've yet to seen one that proposes a system, whereby it would actually work. Most of people mindlessly get excited about the concept, without considering that it's a logistical impossibility, without some heavy changes to gameplay.
I'll leave this thread open for now as, even though Mad Dawg is right, there have been many threads on the subject, all of the others were 5/6 months ago.
This is another feature suggestion though, right?