There's currently an odd caveat with Civs that have experienced little warfare where, outside of unique units, it's often better to sell old units and build them again rather than upgrading them; both because the cost of upgrades gets to be very high, and because the old units will likely have much less XP than new ones in the mid-to-late-game. I was thinking then that having a way to grant gradual, limited XP gain to units might be valuable. This could be done by granting units that are fortified in fortresses and citadels, and that start their turn at full health (to prevent passive XP gain during combat), 1 or 2 XP per turn. The total amount of XP that can be gained this way should be on par with what a new unit would be granted when built, and could therefore be based on either buildings present in the city that owns the fortified tile (which is ideal for maximizing the value of the teocalli and Brandenburg Gate), or it could just scale with era along side fortress/citadel yields.