I think it should not be allowed to upgrade units without promotions so you can't stockpile tons of ancient units and get modern army the second you hit the required tech. This nerfs commercial hubs since you can't get modern army out of ancient 3-turn built units for very little money with 50% upgrade discount card. This makes generals that give promotions more useful, terracota army more useful, encampment production city states more useful, encampment buildings that speed up experience gain more useful, good points all round. As to what to do with units without levels I had idea of turning unit into something useful by moving it into city or districts, this should be available only if unit is outdated and has no promotions: 1) City Center settles unit in the city by turning them into small amount of food, percent of their production cost, useful for new city boost 2) Holy Site converts unit into missionary with single use at holy site 3) Encampment converts unit into military engineer with single use (only medieval units and later) 4) Commercial Hub converts unit into trader, if trader slots are available 5) Industrical Zone turns unit into small amount of production (maybe limit to only buildings) Maybe other disricts can have some uses too, share your thoughts! Also navy units might be exception since navy is pretty weak as it is and you can't get it into districts for above options.