I'd honestly like to introduce the promotion upgrades as seen in DE. Probably double the base XP needed to promote, then introduce techs that promote them all to the next rank for a cost. That way you don't lose auto-leveling, but you can have the techs that introduce progression. Perhaps standardize it thusly:
All C-S classes get the Advanced Rank promotion tech, but each civ only gets the Elite Rank promotion tech for one C-S that's their signature C-S unit. Hoplites, for example, for Athens and Sparta, Archers for Persia, Swordsmen for Republican Rome, etc.