Or just add more quests and put a hard level requirement on them and make people actually progress through all of Luna and the mobs it provides.
Example, (made up)
70-74 (Yeti repeatable)
75-79 (Bear repeatable)
80-84 (Penguin repeatable)
85-89 (Vulcan repeatable)
90-94 (Deadnaught repeatable)
Etc, etc until you go to the next planet. This gives the players a feel of progression through all the mobs in Luna.
This way you don’t have to change mechanics, just add more quests.