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I love the idea of class-specific spells that interact with a wizard's spellbook, a witch's familiar (being much more intimately tied to her spellcasting than a wizard's familiar, a spell to cast spells remotely through the familiar, at range, for instance, would make sense), a sorcerer's bloodline, a cleric's domain, a ranger's favored enemy, etc. More like that could be quite cool.
Of those that exist, instant enemy sure is handy for making a choice you made 10 levels ago and now regret relevant.