Hi!
Personally, I play DF as a non-competitive game and love it because of that (I love SimCity but always go broke). In Strategy games in general, more than the scenarios, I like the sandbox modes because you don't have the quota hunting.
In that respect, I agree with the other people who pointed out that the concept would be a bit odd. While the dark dwarf example may make sense, in general, there is no real score system or anything that could be used to trigger the reward. And in addition, such unlockables are more something for games that feel like video games in the old fashioned sense of the word. You know, some ego-shooters or other action games. I think they go extremely against the flavor of DF.
However, a little bit of randomness in the tech-tree/noble development may add a little fun to the game. Currently, once you have 20 dwarves, you always will get the sheriff and the manager. Once you have a certain amount of coins, you get the bookkeeper, and so on and so on. It would be nice if there was a bit more variation, like, for instance, when you begin a new fortress/reclaim a fortress, the computer secretly determines the threshold for the Sheriff as 15 + 2d6 dwarves and for the manager as 12 + 2d6 dwarves. Or maybe there could be nobles that may or may not appear at all in a fortress (or nobles that are excluding other nobles).
Okay, I have to admit that I am a bit of a fan of unpredictability (that is why I have more fun playing and losing in SimCity than with CityLife - I am always curious what buildings the Sims will build next) and the individualized dwarves with their preferences engraved and random artifacts have so many complex layers that they always surprise me.
Deathworks