I love the idea of retiring the fortress, allowing it to continue existing so that adventurers can visit it. I am sure that will be part of the game eventually.
I had some other ideas that would be great fun, though, for both adventure mode and reclaim.
1) Curse Fortress
Areas around the fortress entrance become Haunted, and all dwarves and animals in the fortress become Zombie/Skeletal. Otherwise the same as Abandoning the fort, or perhaps more like Retire in that items would not get scattered?
--As a bonus it would be nice if there were more types of undead, so that legendary dwarves could become Vampires or Wights or some other more powerful beasty
--As an even bigger bonus it would rock if some dwarf zombies continued attempting to carry out the jobs they did in life: fisher zombies pulling rotten turtles from a brackish pond, skeletal hunters clattering through the whispering trees. Wee!
--In this option, the transformation woudl be literal (Urist Stonepanties would literally have become an undead), and should be referenced in legends.
2) Caveify fortress
Same as abandon fortress, but the fortress is also filled with random chasm, magma tube, and subterranean river dwellers.
3) Dungeonify Fortress
All items remain where they were placed. animals become feral versions of themselves, but remain in cages and chains if they were there before.
-Dwarves are transformed into random monsters based on their experience level, more powerful dwarves becoming more powerful monsters and legendary dwarves possibly becoming megabeasts or demons. Or perhaps nobles become demons, since that is not much of a change anyway. If they were chained or in cages, of course, they remain so. Ideally the monsters should have the dwarf's equipment on and be able to use it: dunno if this means changing the equipment's size automatically to suit the monster, or just cheating and letting a minotaur wear a dwarf-sized Adamantium Chain mail.
-Coupled with adventurer traps, this lets a fortress be designed as a dungeon, right down to what level of monster is defending a particular treasure. Would anyone else spend as much time with this tool as me?
-In this option the transformation would be symbolic: Urist Steelbra would not have become a bronze colossus, she'd merely have been replaced with one. Legends would describe the fortress as having been overrun by a dark power or some such.