My idea is to create a building called “Crystal Forge”, where you can create crystals from things in the surrounding, like gemstones extracted from stones. You need a special worker for that, too. I think it should be a normal blacksmith with an additional skill to be a crystal maker.
But what’s now? I have all these crystals O.O ( not the blue ones from BreakingBad). ^^
You can put one , and only ONE, crystal to every NPC that is under your control. These crystals increase damage, working speed, defence, movement and many other things.
It could be a really cool feature and it improve the possibility to create your own, unique allegiance.