How about to add some bogatir's ?!
A singlemen unit with a VERY VERY high stats !
For example Kiev's bogatir might have a horse and a bow with a little arrows,
Novgorod on foot but with higher strenght, Mongol with a bow with more arrows but high reload time and so on...
Europe nations - heroes...
To produce it u have to build some buildiong or its produce time will take a lot years (18 for example)
I think it would be fun