Mod

Regarding the upgraded Koschei, I think it's unfair – it would be better to make a percentage-based scaling to damage. Otherwise, it's very effective at the beginning of the game, but becomes irrelevant towards the end when damage is measured in thousands.
And it could also be politically conscious and change the Knight and Bear units?)
I had a similar idea myself. You could incorporate a chimney into the hut on chicken legs, like the Necropolis. And depict the Mage Guild as the Spasskaya Tower with clocks. And the Castle as the Kremlin.