First - what is Willpower? I don't remember seeing that stat? Do you mean Wisdom?
Second - why not have Wisdom impact certain spells. Kinda like the Cleric/Wiz distinction in D&D and other RPGs.
Attack/Debuff Magic = Int
Heal/Defense/Buff Magic = Wis
Call...er...I mean Summon magic = Int + Cha
World Magic (stuff that impacts the map) = Int + Wis
This would give "melee" rulers 3 stats to care most about (Str, Con, Dex) and "magic" rulers 3 stats (Int, Wis, Cha)
In addition, wisdom can impact mana recovery and max mana (or max essence, however it works now) and how much you gain on level up ("constitution for MP" so to speak)