Another solution to Xerath

So I heard that you were still planning on raising Xerath's mana costs in 8.1. Seeing as he already has some of the highest mana costs in the game, and both Scorch and Manaflow Band, it seems like it might be a bit of an overstep. Can I suggest having Xerath's passive proc only on a last hit instead? It would nerf support Xerath by forcing him to take cs, and it would make mid Xerath more predictable because he would have to last hit with his autoattack instead of hitting a random minion.