If we can extend this through modding, it'd be definitely cool to see special units like the guard units in European armies or something. I feel this has a real potential. I always found the military organization as it is a bit dull and boring... this will spice it up a bit if Paradox and/or modders can add more specialized units into armies of nations other than that of China/Manchu.
I also would love to see a possibility of adding unit experience, which Civ 4 had (not sure if Civ 5 or Civ 6 have this... don't remember). Experience would be gained through battles, which would make certain units valuable. Unit experience would, obviously, decay over the time, because there is no way experience can be retained without further battles or whatever as soldiers who had fought will eventually die off.
In short, the military organization in its entirety in EU4 (or EU5) really need some shake-up.