I was just theorycrafting a cold themed Spellblade and something suddenly stood out to me. Mage lacks a no/low mana basic melee option. Mana Strike is lightning. Flamebrand is fire and can be converted to lightning. Nothing for cold.
So my suggestion is a cold conversion node on Mana Strike. Additionally make this node convert the Lightning from the Mana Storm node into one of the cold spell procs from elsewhere. Like the Ice Spikes from Frozen Shrapnel in Shatter Strike or the Icicles from Icicle Strike in Enchant Weapon.