Honestly, the theory of "Unlockables" or a "Reward" system get's away from what Dwarf Fortress is.
It's a Sandbox. The true value of a sandbox game is that it gives the player a massive chest of features and tools and says "Go get em' Tiger!"
Minecraft is a sandbox game...do what you want, when you want, how you want with no restrictions on your thoughts or desires by some overarching force that pushes you to perform a certain task.
Garry's Mod is the same. Huge chest of features, no rhyme or reason, meaning no restrictions whatsoever other than the player's own desires or thoughts.
- The Reward system introduces a "rhyme AND reason" to perform a task. Gain 5,000,000 in total accumulated wealth? easy! I'll smooth and engrave every stone in my fortress! not because I'm hell-bent on making the best fort ever, but because I want that achievement/reward.
It's achievements and rewards that break up what a Sandbox game like Dwarf Fortress are all about. They're not catastrophic so to speak, but they are very detrimental to the values the game was built on.
Since this is a discussion, on the other hand you could have player driven rewards...In the form of a Forgotten Beast skull mounted and displayed above your throne room, that Artifact Cotton Candy Hammer that was made displayed in all it's glory in your dining hall...these things allow the player the option, but never explicitly informed that they've gained a reward, the player determines if it's a reward or not.
For me it's outfitting my 10-15 Dwarves in the military with full steel plate...why is that rewarding? I don't know, it's just something that drives me in my gameplay until that point...and then I wander off to one of my other self propelled goals, whatever they may be.