This has to deal with possible 'endings' to fort mode, as in abandonment, digging too deep, losing to a siege, etc. If this is unfinished or if my idea has been suggested, just wave me away!
From what I understand, the possible outcomes of having a fortress are pretty bleak. You either build forever, which could be considered victory, or you can abandon, lose to a siege, dig too deep and all that.
How about, after some extremely hefty prerequisites have been met, the player can opt to 'finish' the fortress? The player would surrender the fortress over to the world and end. From there any future adventurers could find this (still running!) fortress and perhaps interact with it; by trading or by attacking it or what not.
Of course this isn't something needed right away by any means but I thought I'd bring it up quick. It'd be nice to have a positive note on the fortress in Legends. I looked through the development stuff, like the bloats, but I didn't notice anything on this. Could be because I'm horrible at noticing things.
I imagine having an adventurer finding a thriving fortress would be kind of tricky to code. For simplicity's sake it could just have all of the dwarves, named, sitting around and a working trade depot. Actually having them work while the adventurer is watching sounds absurdly complicated to design.
Anyways, just a thought as I was sitting here watching my mason design an obsidian mug artifact. It menaces with spikes of obsidian, which sounds sort of painful to drink out of...