So I thought of this but what if there is a jungler that specializes in anti-healing? They mark people to take very weak DoT damage and while they are taking damage, if they are healed during the duration, stacks up something and depending on the amount they were healed, they get slowed immensely or get stunned.
While your idea isn't a terrible one for a game where a large number of champions will perform any direct healing abilities, a champion who requires the enemy to use their abilities in order to do damage will likely be impossible to balance because of the real possibility that the champion would be useless against certain team comps e.g. any team made of out champions that don't normally build lifesteal or use healing abilities. Your idea would work best in a game like WoW where healing is part of the meta and you can't pvp in large groups or perform raids without some form of sustain.
I worry that this would be too much of a hard counter to champions that rely on healing. Riot quite correctly shies away from hard counters.
