What if you unlocked special hidden spells and skills by doing certain things? Like taming 1000 wolves unlocks a special spell to transform into a wolf or summon a greater version??
The former is usually done in metroidvanias specially by exploring to find hidden stuffs. Mostly upgrades but some metroidvanias and RPGs hide hidden spells and skills also.
Though taming 1000 wolves would tire players and lose the addiction.