Two suggestions:
1. I recently read Dante's Inferno and it occurred to me that the framework to implement something like a tour of the underworld as seen in the Inferno, Homer's Odyssey, the Aeneid, etc., in Dwarf Fortress already exists. Body models, life histories, entity ethics, and, with the advent of cities, large areas in which people meander in a logical way through procedural generation. Conceptually mash adventure mode and Legends mode together and you've already started. In fact, as I think about it, it might actually be easier than cities because Toady could just make one underworld/hell and just have it populated differently in each world, using that world's peculiar (dead) demons or (dead) night creatures to torment the dead. Maybe I'm wrong about that and it would essentially involve a whole new game being coded, but one of these days it would be something very interesting to see, especially as a way of humbling the player relative to the world (Sitsu Murderdeathkill killed 96 innocent peasants in his brief time alive: he gets an eternity in hot magma, melting and re-forming only to be melted again).
Basically think of the Inferno as adventure mode. Dante even pesters people about themselves just like I do in the game.
2. The ability to be a ghost in adventure mode. It is already the case that one can pass the time by pressing . after one dies: I have watched the outcome of kobold raids on my sleeping party this way. So why not let the player be a ghost after passing enough time? It probably shouldn't be a god-mode type thing, but players typically die sufficiently gruesome deaths that it makes sense for them to be able to be violent ghosts. To prevent silliness the ghost could be tied to a very small area, and could simply wait for hapless caravans and kobold raiders to pass by. This could provide a new kind of fun, and add some character to otherwise anonymous areas by giving them a reputation for being haunted. If the adventurer's ghost was allowed to stick around in a manner similar to the rest/retire option, it could also be fun for the player to get a quest to put a scary old ghost to rest by putting an engraved stone near the death site or putting the body in a hometown catacomb.
And yes, I realize these two things are kind of mutually contradictory in some cases, but hey, that's real life superstition for you.