Hey, I kind of like how Hegemony III does it: you can level your unit up when they have gained enough experience, but the customizations aren't just for better combat, but also for economics. E.g. a unit can become a hunter, which will lessen it's ressource consumption. The main feature is this: if youget to the 5th upgrade, then you can create a general and detach him from your unit. So you have just created a new general unit out of your unit upgrades. You can then freely move the general across the map, attach him to another unit (or make him a governor in one of your cites).