Hop, I totally get your point on the Long Wait in a back and forth between two peoples character that are tied together (Grey probably gets it more since he's usually waiting on me). When you're waiting on the other person to respond in one scenario/story bit though, is when you have the opportunity to pick up and write with different ones. Or even go backwards a little bit and write some backstory bit with that tied-down character(s) if you want to focus more on one character/set.
Ideally I think, the ONLY structure required for the whole thing is a Shared Timeline, allowing us to send our various characters arcing through other peoples various characters.
Suppose you just finish a very lengthy co-op story chapter where and another persons characters were tied together very tight through the whole thing and you want a little bit of a break from that intensive co-op. Take a break and write a solitary story with them, if you're writing solitary then, you could even go backwards in the Shared Timeline and do some backstory on your chars that occurs previous to the agreed Shared Timeframe.
So we create a Folder for story threads. Maybe post a character bio thread for people who want to do that apart from story development on them for sake of reference.
All the story threads in this folder, for the most part aside from 'Backstory' development, are occuring roughly within the same Time Period in the SW Galaxy.
This in total then is our Shared Story Universe.
Ideally, after awhile of this, we have a few dozen threads and hundreds of posts and Story Chapters/Bits that come to form this Collective Storyverse of ours.
Just have to disconnect ourselves from the rigid framework of the numbers style strategy game we are used to playing with. Think of it like a Comic Book Universe. Often times the 20 diff monthly titles Marvel or DC are publishing are all loosely or tightly intertwined. Heroes/Villiains are constantly arcing in and out of the stories from one Title to the Other.