The best ideas for what co-op should be were long discussed on the invision GE:S board, way back when. As always, it was fun to talk about but the team is doing MP first ; so it was deemed that if the community got involved with a 3rd party unit doing it, sure why not.
Back then, you couldnt even do it with GE:S (it was always said one day it could be possible via scripting), now with LUA you could probably do it if you are advanced in it and ask for all the right callbacks for us to insert in the base code and understand how to tweak levels using plugins or some such to enhance the "co-op" settings.
If done by 3rd party, it would resemble a little how those CS:S "wc3" servers behave compared with trad. gameplay.