Astelnair is a mod that adds new creatures and much more.

I don't understand. The undead support troop upgrade path:
Wyvern, rock dragon, wyrm - I forgot the name, or dracolich.
After dracolich, there are no more creatures, just like in the second game.
You can use the same texture, or even have a building without a model. You can even customize it if you mess around with the positioning and camera parameters.