I agree with Markavian, something along the lines of Spore seems most likely to me - also because it is easiest to implement. You could base it on downloading other peoples' worlds.
For adventure mode for example, the first simple step would be to have an option to export your adventurer and import him/her into another world/game. You could even make it feel more like in-game travelling by giving the export option only in harbour cities or the like. So what your adventurer would do would be taking a ship to another continent...
For fortress mode alone such content-sharing ideas apparently only make sense if there can be some interaction in between fortresses - but who knows, maybe this is even somehow part of the army arc? Isn't the intend that you can send your armies to attack other sites? Maybe you could send actual reclaim parties to other abandoned fortresses.
Or maybe it will at some point be implemented that adventurers can visit populated fortresses, not only abandoned ones. As a result the interaction in between fortresses themselves could also be richer.
Still, it would probably still be better if fortresses from different worlds could be somehow imported into a single continent. Not something impossible, I guess, but probably not a high priority for Toady...
So yeah, real multiplayer is probably not going to happen. But I could imagine that something like the above could happen, especially if Toady puts together the necessary infrastructure anyway while fleshing out singleplayer (army arc, caravan arc (?), etc).