I do think that a magic triangle of Black > White > Dark > Black could work, though I must admit that the magic triangle, at least to me, that is, seemed a bit pointless when dedicated spellcasters already had sky-high Res to begin with. At least it could work for the purposes of accuracy (+20 Hit & Avoid for triangle advantage), and then have where Black Magic’s four elements (fire, ice, lightning, and wind) would be what he majority of enemies have weaknesses too, White Magic (light element) would be effective against demons and undead primarily, and then Dark Magic have very few who are weak to it, instead often having debuffs and ailments rolled into them, or simply having higher Might. An instant-kill spell or two for Dark Magic would be nice. Generally speaking, you’d be having more monsters and more types of monsters, and they’d be where the bulk of exploitable elemental weaknesses and resistances would be.
Then, have where all three families of magic have support spells thrown in for variety’s sake; Black Magic chiefly having offensively-oriented buffs, White Magic healing and defensively-oriented buffs and ailments (i.e. Sleep, Silence), and Dark Magic debuffs and offensively-oriented ailments (i.e. Poison, Slow).