Field of Glory: Kingdoms has an interesting approach to this by dividing recruitable units into Mercenary, Levy, and Standing Army. Mercenaries dont cost manpower, but have a high gold upkeep and dont level up. Levies are cheap to recruit, but have high manpower reqs to keep and level up slowly. Standing Army units are expensive to buy, but have very low upkeep and level up the fastest and you have a cap on how many you can field. In addition, tribals have lower caps on SA units, but get buffs to levies and other such tweaks to make sure Feudal armies arent the same as tribal or republics armies. Something like that would really be welcome in CK3.