The idea of disconnected play with a central "coordinating" server is not such a bad one, and may possibly even be implemented with almost no change to normal single-player gameplay code.
If that would ever happen, it would need some rules, such as:
-Anyone who joins the server needs to set some flags, such as whether you accept immigrant, caravans, or sieges from other players. (if you can't accept them you can't send them either) and once you turn it on and submit it, you can't change it back. AI caravans and sieges would still happen as normal.
-A fortress can only send one kind of "delegation" each year (be it a siege or a caravan?). Conversely, a fortress can only get one of each type each year. One player siege *and* one player caravan. This in addition to any AI visitors. Now, WHOSE year is that, it would depend, I suppose... if you play 10 years in a single sitting you can send 10 caravans, but if you send them to the same person who plays one year per day, you can expect to wait a lot for all 10 caravans to return. (see below)
-All of these are exactly like an AI visit, it's computer controlled etc, only the actual content of the delegation is decided by the player.
-Since each player builds his fortress in HIS OWN TIME, some kind of balancing act with all the different playing times would need to be done. Say you send a siege to a player and that player never logs in again. Your siege will never get the work done, and your soldiers would never get back. A solution may be to allow any player to withdraw any such envoy at any time unless the target has already received it. (now what happens if the victim "accepts" the attack, saves his game and logs off forever? some kind of hard time limit and it gets called off or some automatic victory is rolled?)
-As above, but with world dates. How can you reconcile that an advanced fortress in year 1065 can attack a tiny fortress hamlet that's just starting up in year 1052? Some kind of handicap, or limitation (you can't attack that guy!) or a limit on the amount of troops (and equipment value) you can send? Maybe some total point value computed in the same way as the embark screen: send one dwarf with lots of skills and masterpiece armor (he's probably still going down, but may inflict lots of enemy casualties... or maybe he'll fall with the first bolt that a dabbling hunter shoots at him), or send many weaker dwarfs with "well-crafted" normal armor.
-Exploits: any kind of cheating to do these things should be avoided, not in a heavy handed way... just try to avoid the situations where cheating would happen. For example, if there was a handicap or disadvantages to a more powerful player attacking a less powerful one, this could be abused into some kind of easy experience/items cheat for the weak player. OTOH simply forcing the powerful player to send a small army to the small fort would not allow that.
Many more of these things would need to be ironed out. For all those naysayers, remember that this is pretty much unchanged single-player Dwarf Fortress, except with the possibility that someone else may decide to send a custom (balanced and non-cheating?) caravan/siege your way (and vice versa). Or you can just disable all interaction and simply "visit" other fortresses without interacting (how about in adventure mode?), or simply decide to turn it off entirely and play it with your own randomly generated world in your hard drive (status quo).
Anyway, even if this never gets done it's still food for thought. Maybe a nice brainstorming for anyone who wants to make his own "Klingon Space Fortress" clone in the future.