Hi!
I am not quite sure whether it fits with your intents, but if you are willing to take requests/suggestions for fortresses, here is something:
First, gen an umodded world with lots of civilizations (like 60 on a SMALL world, 80 on a MEDIUM) and lots of forests running to at least year 1500 (as long as possible, basically, but genning grows slow over time). Have all variances except volcanism relatively low (about 100) so you get a lot of big subregions. The aim of this is to get a world with a lot of historic events (lots of forests help elves survive so they can fight many wars with humans). Ah, do not cull historic figures and do have history hidden. Also turn OFF history revealation via engravings in fortress mode.
Pick a site in at least one large subregion where a lot of wars have taken place. (If you get more than one main subregion in your embark site, it is even better).
Make a fortress without any underground constructed floors or walls and double engrave ALL walls that are reachable at one point or the other (so, if you place furniture into a room that blocks movement, double engrave beforehand).
For a special challenge, make the fortress a 3- dimensional labyrinth of corridors with small and medium rooms hidden in the depths of the labyrinth (nice for testing the pathfinding AI :) :) :) ).
Surface constructions can be as you like them (towers, houses, none, whatever).
Finally, abandon that fortress regularly (that is, do not lava-flood or otherwise destroy it) and upload it to the file depot.
Option: Abandon during a siege.
Basic Idea: The bigger, the better as there will be more engravings.
Option: Hide interesting rooms/secrets that adventurers of other players can uncover in the fortress. (Don't tell people about them, rather let them report here when they found them)
I know, it is not much of a challenge, but I think it would be fun for people to explore your maze and uncover the history of your world that way. And maybe be surprised by what spontaneous ideas you have put into the fortress (the design definition here is deliberately vague).
Deathworks