Forget Succession Fortresses.
With fortress retirement looming on the horizon, we can expect soon to be playing Succession Civilizations (taking turns building strongholds for a nation till its dominance is established among the rising and falling civilizations Toady's been working toward), playing as adventurers blazing into those megafortresses people upload to the DFFD, or -- once the army arc materializes -- waging war against computer-controlled but player-crafted fortresses/civilizations. In time, assuming it's possible to take a retired fortress and reclaim it to build onto it or repair it, it should be feasible for players to take turns with a world, each turn lasting so many years fortress mode or so many days or months adventure mode, and the only rule being that each player must play the same civilization each of his/her turns; they can rebuild a fortress that was damaged by their opponent's attacks (whether army directed from afar in fortress mode, or adventure mode marine squad), or build another one right behind it to stop the enemy's continual advance into their territory, or launch a counterattack on the enemy's fortresses. Think of it like a combination of Legos and Risk! And that's not even getting into the possibility that the underground caverns could, sometime down the road, be as feasible a map to war through as the overworld itself (which is quite a real if long-term possibility, judging from Future of the Fortress replies about the possibility of defensive sites to deter goblins between the Dwarven Deep Sites and the caverns, and about the long-term need to allow sites to overlap x/y-wise while above/below each other for the sake of animal-men and such).
The possibilities are going to get so interesting that it won't matter that you don't play against the other humans realtime. And the most crucial mechanic to making it work is fortress retirement, which is very close to being a reality already. In the short term if nothing else adventure mode is going to be more interesting for "competitive multi-player/challenges" with massive player-built fortresses being still functional and self-defending in retirement, and in the long term I think we can hope for the above ideas about taking turns waging intercivilizational war above and below ground at least to be on the table.