then there is the discussion of historical accuracy that pops up every time. In that regard there is a risk, that every form of implementation either falls into the trap of having rather pop culture like factions or that the game will be criticized by people because the faction does not meet their pop culture influenced exspectations.
Especially "Far east".
Also, as much as I like them, we do not need a far east faction in every game that somehow deals with medieval things.
Lastly, every faction added now, would be one absent in Warband - meaning they likely loose for some reason at some point in the 200 years. In contrast to the actual factions, there would be no trace of them.