I had an idea the other day about a cross collaboration story. Basically the story is set on or earth where a group of scientist accidentally discover alternate reality travel and are invited to join an alliance of sixty eight other earths though some have chosen other names for their planet. Only catch is that the portal only allows organic materials to pass through so no clothing or anything. The main story would be a short introduction and lead up to the adventuring traveling through the portal. But instead of using chapters links would be added for what ever world they traveled to so each author could write the story their way with no limitations with a link back to the main story on the first page. The benefit would be that the attention one story would draw would lead back to the main story and then to the other writers. Worlds 1NC35T- A world where incest has become common practice. L35B14N- a world populated by all women who breed through Parthengenesis. TR4n5- A world populated by Futa females who have a feminine appearance but have both male and female sex organs. Etc, etc..
While sounding nice, I'm afraid in terms of story writing, it would just turn into a few very different stories without connection. And then, frankly, what's the point? The best collaborations are inspiring each other or let ideas from other story threads reappear in yours. Maybe do something like the travelers can hit "escape" any time, but are then thrown into another world at random. And... more. The more interconnection and interaction the better.
Also, having the portals follow "Terminator Rules" I think is not especially good, as it prevents interplanetary trade and warfare, which leaves little incentive for anyone to join or have joined this 68 planet alliance... (Well, unless you count slave trades, but that would just cause us to try to find a way to invade and liberate the other worlds, not join them...)
When I read the title I thought sweet a world where you 69 your partner to impregnate them. Then reading the story idea I'm thinking this is going to need to be a very long...long...long story. My view with stories is KISS it. If it's not I don't read it.