This probably belongs in Homebrew/etc.
Thoughts...The Forsaker PrC from old 3rd edition Masters of the Wild was an anti-magic hero who powered its special abilities by breaking or destroying magic items, which could be at least a base for how hungry your spell-suckers get. The Spellfire feat and Spellfire Warrior, likewise, involves thaumovores.
Obviously their eternal hunger for magical energy should be blessed with suck, so figure out some degenerating stats and a scaling "hunger" as they go up in level. It would also fuel their abilities, of course, but you don't want a player being able to eat a charm person and turn it into a Dominate Monster, or even another charm person, entropy demands its due.
They should also be able to kill normal people for juice, but I'm thinking instead of permanent con drain (like vampire) go with Charisma damage, since it ties together well with imagery of the brightly-burning soul being snuffed out to feed magical energy.
They should glow, I know we have the whole Twilight Sparklepire abomination to work through but it makes sense to have a magic-eating critter have glowy veins or the like.
I really like the idea of modified spellfire wielder, since it makes sense that a woman who drinks magic like water should be able to eat a fireball if she's quick and prepared to catch it.
