In games, RPGs in particular, it's hard to give a good reason to fight/farm a particular enemy or area of enemies. Puzzle & Dragons does a good job of giving reasons to farm particular dungeon, as bonuses of x1.5 drop or enemies that drop particular evolution material. But in a game like FF7, there is very little reason to fight one enemy over another outside of the amount of exp it gives.
I have an action rpg for mobile where players can choose to explore different dungeons, unlocking harder ones as they complete easier ones. My game only has class experience and a single currency, which is in turn used to purchase more classes or more abilities for a class, as player incentives/rewards. So now I am struggling to find a reason for the players to return to a particular dungeon outside of the chance that they are leveling up a weaker class and need to start at the lower dungeons again.
In a game with two small player incentives, what can I do to incentivise players to return to previous dungeons? Should I introduce farm mechanics like "to unlock a class you need currency and this shopping list of enemy parts/items"? What games have you played that you remember giving great incentives to return to previous enemies/dungeons?