Here are a few ideas for possible Polytopia features. I just came up with them randomly, and I think they're kind of cool, so I decided to share them here. I'll add more here as well.
Merchant: A unit that can be used to make trade routes, like roads can. If a merchant unit walks from some cities in your empire to your capital (or vice versa), a trade route will be formed. The merchant costs 5 stars to train and has no attack or defense. Its health is 10. If a city, that has only Merchant-made trade routes between it and another city, is captured, the trade route will not come back if you take the city back. I like the idea of having to protect a certain unit like this, it makes a unique playstyle.
Angry Villagers: There could be a small chance - changing depending on difficulty - that a village will reject you when you try to capture one. Then, a Warrior (50% chance), Archer (25% chance), or Rider (25% chance) will appear in the city, using the "model" textures for units (black shirt, no helmet). Only one unit will ever be trained in each village. If you kill that unit, the village will finally be yours.