Hm, you mean besides having the most powerful spells of all?
Immunity to cursing would be adequate imo.
Would also be useless if you are not using normal weapons, since curse only slashes your hit chance, which is something a mage doesn't use anyway.
I'd rather they get a small chance to bounce afflictions back to the attacker (i.e. a 5% per witchcraft point chance to not be affected by weak/cripple/bleed/etc, and if the status is prevented by this bonus it tries to apply itself to whoever used the attack)
OR just when you succesfully save against a debuff, the attacker gets debuffed instead. Witchcraft is all about inflicting various debuffs, so let them get some spiffy bonuses for that.
Or maybe grant something blood-related. Since blood is Witchcraft's 'element' (you get -1 affinity bonus when standing on blood with the elemental affinity trait), why not do something with that. Enemies in blood get penalties to resist your witchcraft spells or something. (or you give enemies penalties to resist your spells if you're standing in blood yourself. Maybe alongside a blood sacrifice spell where you harm yourself and create a blood surface to gain some kind of benefit, or hurt an enemy more like a reverse ranged vampiric touch)