You guys are right, L4D isn't open world; but i mentioned it because their campaign mode is probably the best way to do objectives in coop. KF converted to a campaign would have players role playing a story through several KF maps transitioned by safe houses. That seems by like an open world compared to the sandlot experience we have in KF. Sure the progression is scripted but usually there are different ways do accomplish things each time you play to always make things interesting. That's one of the reasons that gave L4d so much replay value. i am still trying to get used to there not being a sequel.
Anyway, as for a open world game proper, i suppose it would be hard to do without making an entirely new game like you have all said. I don't think that it would be impossible. There are many creative minds out there and hopefully this thread will be food for thought.