How about Trolls? Or some kind of fallen human. Maybe an Orc-type being? Something that could exist along with the rival empire.
I did a bit of planning and design yesterday, and here's what I'm currently looking at for AI settlements. Some of these may show up as built-in mods, rather than in the core game, and some may be cut.
- Bigguns - Large, ogre like, giants.
- Diggers - Underground dwelling mole-people (probably tied to a post-release update that adds an underground layer like Age of Wonders).
- Mushroom People - Durable, peaceful, fungus people.
- Villages - Villages of fairly peaceful humans.
- Hill People - Primitive, aggressive humans that like raiding neighbors.
- City States - Advanced human settlements that can grow and expand (these are the rival empires).
- Pirate Cities - Coastal human settlements that raid on the seas.
My vision for the diggers is to have them inhabit an entire underground world. There would be caves dotted along the world that lead to the cave "layer", which is effectively a second world map. By travelling underground you can exit through a cave in a different area on the surface.
It could add a neat dimension to trade and warfare, with armies appearing from caves potentially well inside your empire.