I for one think it's not that great of an idea. I'd rather not see the game turn into a lame GTA clone where it only pretends to be a sandbox game but in reality there's little to do besides someone else's idea of a fun story.
Besides, we already have all these sorts of challenges. The wiki has a huge list of them, and to me, the only difference between that and this is that you're taking away the challenge of finding a suitable site and getting your fortress up and running all on your own. It's like DF made easy, sort of. Not that spending forever in worldgen is fun... the only reasonable condition I can see to include anything of the sort would be as a newbie tutorial. Play quest 37, creating a soap industry! Be the first to create 200 bars of soap!
I like that we have to create our own fun and I personally think it adds a little to the community feel of this single player game. I've never seen another game where people discuss so much of the crazy stuff they've done (and in such a friendly way).. even in highly moddable mainstream games.
I'd also worry that people would treat it like world of goo. Do all the prepackaged challenges and then declare the game over and go do something else. I can't imagine the game giving you some of the crazy awesome stuff players think up to do (like merperson breeding programs), so why not just stick to interacting with other people while thinking up your own ideas rather than wanting someone else to hold your hand?