A few basic propositions to work from:
By the time the game is finished, the Sburb players have managed to restore the Earth, though the population is greatly reduced - between one and three billion. This has been explained in the history books as the result of a plague.
All of the human players, and all of the trolls currently alive in Homestuck, survived. The currently dead trolls are all negotiable.
The restored US government knows about the existence of the trolls, but there are initially no more trolls being born. Kanaya is working to spawn a Mother Grub somewhere.
Until this happens, Rose and Kanaya (with Jade's assistance) can use ectobiology to hatch a very small number of trolls.
The name of the AU is "Double Jeopardy". This comes from the fact that the Earth will be taken as the opening stakes of a second Sburb game, played (at least in part) by the descendants of the players of the first.
Some specific suggestions:
The game should be played by two sets of players - the first born before the resurgence of the trolls, the second after.
There should be either six players or twelve in each set (12 or 24 in total). This will depend on interest. I'm leaning toward six each.
Across the entire group, it will be split 50/50 male/female and 50/50 human/troll.
Neither group will be able to complete the game on their own, but will need to join forces in order to win, much like the post-scratch trolls' Red Team and Blue Team, though with a greater degree of initial isolation.
To keep things from getting too redundant, I'm going to suggest that every pairing only have one human and one troll ectochild involved in the game at most, and that the session players are no more than 50% ship-kids.
Another consideration: There has to be a way to keep the canon players from taking over control of the session. We can work this out however we want. However, in any case, it's likely we'll need people to RP as a small handful of canon characters, in addition to OCs.
All of these are flexible at the moment. I'll make some of these early brainstorming posts private once we get the ball rolling.