Hmh... Actually, I don't have trouble seeing it happen. IF done in a easily implementable way.
Hot-seat is clearly out of the way and even real-time gaming on two fortresses is a definite no. The processor power and amount of data that are necessary to transmit would be way too much.
What I DO see is an asynchronous social linking system. Or to put it into less pretentiously intelligent words: People play their own fortresses but can interact with each other in some fashion. Like, you got a friend who plays DF too, link your game up with his and interact with him indirectly. Here are a few possibilities:
Caravans - Select a few goods, either for open trade or with demans for specific other goods, built a wagon, assign trader dwarves and send the caravan away. The other player will soon receive the caravan and may trade with.
Passive sieges - Assign military dwarves to a task force, select the player and send them away to wreak havoc on him. If they've arrived it's already too late for him to disconnect the game!
Support - Got a siege coming in? Ask a player to send reinforcements.
Things like that. The only real problem I see is DFs moddability. Any given mod that'd differ the player's client from another client would completely screw up the game.
Naturally, DF DOES have more pressing issues now :V
But how do you synchronize fortress years. I mean, it is all fine and dandy sending out a caravan from Fort X in the early spring of 254, but having said convoy arriving at Fort Y in the late autumn of 251 might sort of kill the continuity as both fortresses would be required to be built in the same world.
True. Though the way to solve that is to have the forts be on separate worlds and have some kind of gate/portal/negative space wedgie to travel between the worlds.
There could be problems with the combat, the AI is stupid.
How will your 50 axe lords beat a lava sea or a walled in fort?
Either a lot of AI would be needed, or you'd have to control a bit in sieges.
That, or you never siege because a single wall keeps everything out.
Siege AI would need work, and maybe better siege mechanics, like a siege tower or digging. And if we have full vehicles, then there would be potential of a massive iron monstrosity that your dwarves could ride in till it could path no more, or something like that. For internal drowning/lava/weapon/etc traps, well that would be a bit more complex in figuring out how to defeat for the AI. Also, ideally, not all multiplayer interactions would be siege. There could also be trade, which would provide some incentive to allowing players to be able to interact, especially in up coming versions, where resources will be more scarce. Could also have a setting when you dispatch your army if you decide to siege. Bloodbath rush, or 'starve' them out. First option does like a goblin siege. Second option, the dwarves sit on the edge of the map unless something gets too close or a caravan or migrants arrive. If resources are more scarce, then trade becomes more important. Sure, most likely a dwarven fortress will be entirely self sufficent food wise, but other resources, such as those needed to fullfill mandates, complete artifacts, and several other things that if ignored could hasten the infamous tantrum spiral. Heck, could even have it to where as long as another player's army is sieging the fortress, all dwarves get an unhappy thought, to discourage turtling against players.
Edit: Had forgotten to respond to some of the posts here.