I would specifically tie a stacking on-hit debuff to every enemy hit by a fire spell whose mp cost is discounted by an umbral heart. That way using Fire 2 to soften up enemies before you Flare can come back into vogue.

And, I would make the debuff interact specifically with your other actions - maybe it’s vuln to your next Foul spell, for instance - rather than buff other players. Otherwise there’s be an incentive not to wipe/consume it for others’ sakes.