It just can't be done without being half-assed, I mean it's even in the word, they would be generic.
Medieval II has too many limitations already that forces us to remove a lot of faction individualization. What you are essentially suggesting is that by adding more factions, then the few things that we managed to compromise to achieve some manner of faction/race personality, would be lost. Every faction would essentially become generic and boring apart from faction symbols and units to differentiate each other.
I mean I understand that you as a unit maker would want to make a load of them, touch basically every faction and race present in WoW. That's great initiative, but the rest of us modders that have to deal with 2D art and general modding just understand the mess that would create