I'd love to see a spell based off of religious frenzy like the bacchanalia of Rome or the Wild Hunt of Northern Europe. It would either be based on Idon (bacchanal) or Asketi (Wild Hunt). I don't have much more than the idea, but perhaps it can be a directed-attack spell like Starlight Sphere or Ball Lightning, a summoning spell (there's plenty of Asketian creatures to choose from, and Idon created the Dark Fae), or a berserk-like booster spell that's powered off of devotion or soul.
~Thilan