Malignant cells draining quanta is the worst idea in the world. They would instantly shut down your opponent. The disincentive from using it on yourself is called "the enemy" -- you just kept yourself from playing any creatures at all, and the opponent can deal more than 23 damage per turn, I guarantee you that.
And Carcinogen already infects as per your suggestion. ???
On another note, while it's good that the AI is trained to target single Malignant Cells in an attempt to keep them from spreading, it's kind of silly to target them with Reverse Time...and then play them out of your hand.