Well, having a theme or not having one won't affect the process much on general... I mean, if you would have a theme, some people would have easier time deciding on a script... then again, without a theme, people would have less limitation and can be more creative. So, no good or bad choice here.
I am not sure why you need participants to write PM, but I would say that for this there shouldn't be a fixed list of participants. I mean, sure, if some people would state that they would write something - that would be nice, naturally. But this should be open with no fixed list, cause some people may decide to join while it would be in process... plus, as I said, writers would have to spend time searching for a willing artist, so even if they would write to you now, they won't be able to guarantee they would find someone to pair up with...
Minor point, now that I think about it - make sure to remind people on the actual project then, when you would have one, that they can use it to look for people to cooperate with. Well, that's pretty self-obvious, really, though.
You may also want to think about the finished product. Like, okay, you start this thing, you set a deadline, you get to a deadline... and then what? In choruses, in the end we have a finished video. In contests we have a voting of sorts and the winner. In DC's writing thingies we have a book for everyone to download. Now, here, obviously, the process and the 4-panels we would get are the main point, but if you want, you may want to think about making something with the finished results. Again, no bad choice here - if you would want to do this, just pick whatever you want.