Re: media creators, I participated in megaflare_ff (and still haven't posted, as I'm waiting to hear back from my artist), which uses a similar system to polybigbang. Writers posted summaries, then artists went through a first round of claiming, followed by a second round if they wanted to create for more than one fic. This was set up so that not all fics necessarily got a media creator, but you could mandate otherwise.

Also, I love AO3 collections, yay! And I would love to participate in a second round of whatever word count, though my schedule is completely up in the air right now, so I have no idea what dates would be best for me.