I would think this has been submitted before, but here is my idea anyway:
As Toady is working on goblins attacking towns in adventure mode, I think doing the same thing with dragons would be great (and probably simpler than goblins). That is, dragons should terrorize towns, basically burning and killing everything. Then, our hero would arrive on site, and for "service," everyone would scream that there is a dragon currently sacking the town. Then, the hero would approach the dragon, and they would duke it out in the streets. Dragons would be in town perhaps one day per week, and otherwise be in their lair.
My conception of dragons is that they would regularly siege towns for treasure or fun, perhaps needing a virgin sacrifice or something to calm them down. Right now, lords and ladies do complain about the beasts terrorizing the citizenry when you talk to them with "service," but it would be nice to see dragons in action. I haven't seen a dragon in adventure mode yet, but I think they live in lairs or just wander the wilderness - and that's kind of boring.
Perhaps dragons could pick up loot as they go, and create a horde in their lairs. (How do fantasy dragons create their hordes anyway? Clawful by clawful? Seems like it would be inefficient. They probably steal treasure chests, I guess.)
Maybe dragons could take up residence in the city keep, or the catacombs, or a temple, or the dungeons (please, not in the sewers).
I want this more than smart goblins and bandits, by the way.