from my guess, The witch and Victoria have the most potential to teach us spells and I think most of them would be self-defense spell or stat-boost spell in general, I don't think they would have a reason to teach us destruction spell beside Nameless.
(SPOILER ALERT...maybe) I have a theory that if we served Nameless we could learn spells/blesses of destruction but also could lead to bad ending forever (since he blocked us from changing decisions)
But i also think we can learn harmful spells from new character or just by looting stuffs, it would be cool too. And I don't think if we could learn something like meteor that would break the game (unless we turned into the final boss lmao). Seduce spell also sounds great, if Hyao let us use it outside of combat and inside of combat (but i'd prefer it to give "charm effect" to the opponent rather than a convenience lust potion spell to raise the libido or love potion...)