I had an idea for the game. What if combat units could duel with other to gain experience even when there are no current threats. It probably would be less than normal combat, but would be even more helpful in late game combat to allow lower level combat units to strengthen and not get slaughtered. The complication to this could be that after a duel the combat units are injured, therefore it isn’t just free experience.
Edit I know that training dummies have been suggested. However, I think that this system makes more sense. The units would be practicing through actual combat with each other, not hitting a piece of wood.