Now that we have burrows, might it not be possible to do a hot-seat multi-player game of sorts where you embark on a fairly large map and just split the dwarf force and their supplies in two, and create two sister fortresses, each ruled by one player? Step-by-step setup:
• Set embark points to twice the usual amount in the worldgen parameters (yes, you can do this now
!). Split it in half an have each player spend his or her share on skill points for the dwarfs and gear. Flip coins or whatever for the fourth dwarf.
• Upon embarking, divide the supplies and define two burrows, one for each player. Assign one player's dwarf to one burrow, and another player's dwarf to the other.
• Take turns digging out tunnels, setting up squads, and doing whatever else you need doing as the two fortresses chug along.
• Handle trade between yourselves and caravans as best you can. Don't think war between the two fortresses is possible, but who knows, might be possible to mark your own dwarves for kill orders (would make noble management far easier
!).
The only stumbling block I see is how to keep supplies in the right fortress -- I don't see a way, short of building a wall or something, to get the dwarves to understand that
these supplies belong in the northern fortress, while
those supplies belong down south. Should work out fine after the early-game, though, as the burrows will keep the dwarfs from running out to get your ally's newly crafted steel swords or whatever.