It shouldn't require "A COST." And I did read your post this why I disagree with you. If you want a cost just increase the amount of MP used. But you want something more detrimental.
Runefencer has the most powerful AoE enmity tool in the form of Foil. Usable more often than flash that one spell is WAY too OP imho. If its cast cast was double that of flash then maybe, MAYBE it would be in line with how weak flash is currently. An AoE even without a potency buff would allow Paldin to close the ground with Runefencer.
And I'm still not going to leave it alone. GIVE PALADIN URIEL BLADE!