You can already level up subs with broken drones and whatnot, but it doesn't do anything, nor does the level persist when you king your heir (the level is reduced back to 1). Your king gains stats when he levels up, so why not sub males? Especially since they could be your next king. This could help people raise stats for their heirs and, if this is considered, subs could gain exp when patrolling just like lionesses gain exp when hunting. Sub males occasionally gain stats from patrolling, but in my experience it's only 1-2 at least and probably 3 at most. A majority of the time my subs don't find anything on patrol.

-What I'm suggesting-
*When your sub goes on patrol, he'll gain exp and probably the ocassional stat just like lionesses do while hunting
*Once he gains enough points to level up, he gets a boost of a few random stats just like your king
*Rinse and repeat

-What I'm not suggesting-
*Sub males keep their levels when kinged. While it would be nice to not be caught in the same 4 zone loop like I am ( temperate savannah, shrubland, tropical forest, dry savannah). I can see people complaining about how it might make the zones too easy to reach, since you could just buy a high level sub.