Part of the reason I struggled with deciding which covenant to settle with for my characters is that none of them matched aesthetically with my core class abilities.
It’d have been nice if choosing a covenant unlocked the option of using alternative particle effects / animations for your class abilities, even if it’s as simple as a color palette swap.
Making all my Shadow Priest’s abilities blood-based, or having my Ardenweald Paladin’s consecration glow a nice faerie pink would’ve been a nice option to have, and would lend itself to the fantasy of embracing this new anima-magic. Of course it could go deeper too with stuff like Druid forms, Demon Hunter wings, and what have you.