First, this will never happen, but we can dream.
riot adds new items to the game for dominion
Kassadins Bane - unique passive, whenever kassadin warps within 1300 range of the bearer, a debuff is placed on him for 13s that puts a surprise on his travels through the void. If he tries to rift walk within the debuff period, he will rift walk through the void into the birthplace of the most evil creation devised by gods or men. A teemo void shroom,the holy evil comes calling home, damage calculated assuming a void teemo had 1000 ap and bft and 200 m pen.
In short, he tries to warp in and out, too soon, he get's melted by something equally cheesy.
Teemo's bane - unique passive, whenever the bearer walks onto a shroom, a black hole appears above teemo's head, and from this darkened void comes a riftwalking kassadin enraged at the indignities suffered from an endless sea of void shrooms, he silences then does that void wave thingy. If more than one shroom is walked over, simply add to the black holes that appear above teemos head and launch a vengeful kassadin. Should teemo be dead when the bearer walks over a shroom, the black holes will stall until the moment teemo resses, and they will all unleash at once, often dealing so much burst damage the teemo is blasted to dust before the fountain can heal him back to life... so sad.
And there you have it, that would be a solution. Matter and anti matter, two holy evils set loose upon each other. It's kind of like in the wheel of time series when mashadar was set against the dark ones taint on saidin (the male half of the one power), their mutual hatred annihilated each other. And so it should be with teemo and kassadin.