I enjoyed your post so much OP. Yes I'll also agree with an Ultima spell! It absolutely fits the fantasy and you're bang on correct with the perception of blm. For me, it doesn't exactly have to be called Ultima, it just has to feel like a powerful, signature spell that transcends foul. If it has callbacks to previous FF games, then that's even better. Anything that builds up to that fantasy, like a crazy spell animation, a unique staff casting animation, fat potencies or mechanics you'll have to go through will benefit it greatly. Currently Blm mostly spams fire4, with foul every 30 or so seconds. Imagine if an Ultima-like spell could be used every 2 or 3 mins? And a unique, new Blm mechanic could tie into how it's able to be cast.
In fact, it'd be so powerful that it can't even be swift/triple casted, but to compensate, it gives you a brief 3 second immunity to damage. If so then that would definitely perk up a wizard's hat.