A while ago I had an idea for a crazy fortress. It would feature things like a maze of bridges (at some point you'd have to jump off one to another to get through), level puzzles, and a treasure room full of awesome stuff any adventurer would be glad to have at the end. It would climax with lava being released into a trough that runs under the fort that would collapse it in sections as it lit up flimsy wooden support columns, resulting in the entire place being wrecked as you triumphantly walked away.
Of course, I never finished it. Making dwarves do anything that involved channeling out multiple z-levels one on top of another is more of a pain in the ass than I'll ever put into this game, but thinking about it, most of my fortresses have some themed construction in them that should look interesting. Adventuring in these places in adventurer mode could be lots of fun.
Of course, even smaller worlds are way to big to make this an enjoyable experience. You'd have to make a whole world full of these places to make it decent. I don't think I've ever had a world where I've made more than one fort. I'm usually looking for an ideal location.
That said, I think when I'm done with a fortress, if there's anything of note there I'll either make a point of starting a new fortress on that world, even under less then ideal conditions, or post the save somewhere public so that someone else can leave their stamp on the planet. If anyone knows of others doing this, please point them out so I can take a look. It would be all sorts of awesome if we could get a dozen or so sites on a medium/small world that are really interesting. Vanilla adventurer mode really leaves something to be desired, so this would be great.