This is a small idea to give each demon more of a unique identity because the only things about them that you can't freely customize right now are their resistances and stats.
Each class of demon has one of its default abilities marked as "intrinsic". This makes them unable to discard the ability, but also boosts the effectiveness of the ability to a degree that is unique to that particular class of demon(the same ability can be intrinsic on multiple classes of demon, but they'd have different bonuses). So goblins might have more evasion from Mob Cover, slimes might have a higher bonus for Malleable, etc. I believe you already had something like this in mind, but this is just another possible path. Also, uniques could have two abilities marked intrinsic instead of one.