Sounds like a very interesting idea! Very meta!
I can't make any promises, but I'll check back in when it starts up, and if I like how it's going, I'd be glad to participate! (I'm not one to brag, but I am a bit proud of my world, and I think it'd be a pretty interesting one for them to wind up in.)
Personally, I'm not too sure about note 3. While I'm certainly willing to work together on a collaboration, I feel like it will look disjointed if some characters are drawn by one artist, while others (and the background) are drawn by another.
You might consider communicating with the guest participant and plan out what will happen in their universe together, instead of people trying to work on the same thing independently of one and other.
Or maybe it would work, but that's my intuition on it.
As for titles, how about "Multitraverse"-- a combination of "multi-verse" (a group of different realities within a single continuity) and "traverse", meaning to travel.
Well, it's not great, but I'm just brainstorming.
Again, it sounds like a cool idea, and I look forward to seeing how it turns out! Be sure to let us know when it starts, okay?