The race which SO needs a split is Slavs.
This is an absolutely an umbrella civilization, Slavic players literally do not feel that it reveals their peoples. Russian players do not see it as a sufficient embodiment of Rus’, and representatives of other nations do not like the fact that units speak Russian. The Slavs reflect the RUMANIANS in Dracula campaign. In addition, it is technically quite weak, has a bad team bonus, has almost no imperial period and in any case needs to be reworked.
I already wrote a topic about this:
In short, it should be divided into: Rus, Ukrainians (Cossacks), Vlachs (Rumanians, they are partially Slavs), Serbs, Balkans.
As for the rest. Maybe it is still possible to divide the Ottoman Empire. Personally, I would like to see civilization of Armenians, but the rest suits me fine as single empire.
As for civilizations of Mongol type - theoretically, of course, it would be possible to add the Xiongnu (they are enemies in a couple of campaigns), but I very much doubt they could be any different from the Mongols.
I also think there should be Swedes in the game, separate from the Vikings (just like there are Italians, separate from the Romans).