[Suggestion] New Mage Keystone
Cunning tree _Skyscryer's Ultimatum Using up to 30% of your max mana will give your next abilities X% of your max (or missing) mana as additional magic (or true) damage for Y seconds. (Z seconds CD). _
Here you have another viable mastery for mages so that they might decide to build a lot of mana rather than mana regen, numbers are not my thing but I think the idea is balanceable.