There is one damage element we don't have in League
Long duration % missing health dps. Example: deals 3% of the target's missing health every second for 10 seconds. We do have missing health dps in the form of Fizz W. Neither its duration nor its burn rate is something to behold. We have plenty of abilities that deal more damage the less hp the target has, but this is the same versus everyone.
This element would be thrown into something that would behave like Curse of Agony. Anyone who has ever played WoW knows this one; it starts off meh and ramps up over time naturally. It's freaking awesome.
Long duration % missing health dps would basically be the best burn debuff while being relatively easy to fuck with through heals and shields.