DeletedUser15432
Maybe something along the lines of levelling each units "special ability" then? Personally I would enjoy to see purchasable upgrades for units. Such as a sniper from sniper camp is simple and basic and then you can pay for upgraded scope, upgraded rifle and things such as that however to be able to purchase those upgrades that unit must have a certain amount of kills or reach a certain rank?
Such as ablative aplique armour for tanks, HESH rounds for artillery etc etc