Whoa. Glad to see replies.
Optimist, I'd "GM" insofar as I might keep things moving in a general direction, but the idea is that there isn't much world building ahead of time. Improv comedy is a good way to think of this - there's a premise to start with, the participants add to it without contradicting anything. Roll with the punches, so to speak - literally and figuratively.
Not to say our characters can't be complex and possess actual motivation. But. You know. Whatever.
I'm gonna shoot for 1 more participants (assuming you two are still in - excuse my absence) before making a real thread, but start thinking up...stuff.