Maybe you should add a potential limit for each creature, for example, when the creature reaches lvl 5 it reaches the level limit, unless the person buys a stone to break the limit for 100 Zing from an NPC, but to access this npc you need a membership of 500 zing per month to access this place. So at lvl 15 you need 3 stones, at lvl 25 5 stones... Maybe the first stone the player will get for free by completing a mission...