OP but pretty sweet idea. I would just say killing an enemy with a melee attack increases the damage of your next melee attack or even makes you go berserk and increase melee damage for a short period of time. Otherwise I could litterally punch for days. Also, if Shoulder Charge counts as a melee kill its OP, I would make it Subclass specific and either go with Storm Fist, Disintegrate or Sun Strike.