On running multiple fortresses and/or adventurers in the same universe at the same time:
Have you given the idea of running multiple forts at once much thought? Do you expect this to be possible from the whole "run the entire civilisation" thing you were talking about before, or do you expect to use a different mechanic?
Will the player be able to visit a functioning version of his fortress in adventure mode, instead of being restricted to only visiting abandoned fortresses? Perhaps the player will be able to temporarily resign the controll of the fortress? Or might this also somehow be covered in the "run the entire civilisation" mechanic?
Regarding the idea of the player being able to access a quasi-legends-mode while in Fortress or Adventurer mode:
Will the player be able to easily see the state of the world and all it's wars and so on while playing, and perhaps before embarking? For example, I've heard that while WWII was being fought the papers regulaly had little maps showing where the current frount-lines were and so on. I just think it would be rather cool to be able to have a look at where a war is currently being fought, and then deliberatley go and place a fort just behind the lines, to support the army perhaps. That would give the fort a more "real" purpose, because I doubt that the mountainhomes would, in reality, tell a bunch of dwarves to go out to a random spot for no reason at all.