The Counter to Untargetability

I'm sure somewhere along the line Riot has thought this over before, but just in case it hasn't been reviewed, I wanted to bring forward the idea to create a champion or integrate into a rework an ability or passive, or even rune that allows for damage to be done to untargetable champions. For example, a mage possibly from the Shadow Isles who has an ability, say it was named Spirit Pulse (because ghosts), and it was an AOE ability much like Skarner's, but with a larger area. This ability, as it would hit untargetables, would cost 100 mana or maybe more and would deal damage to all enemies in the area. However, I realize the chance to take down champions who are not supposed to be killable is very strong, so possibly make the ability do say only 30% of the total damage and be able to target only champions so it could not be used for wave clear and could only pick off low hp untargetables. As a rune, it would be a keystone that allows you to target champions that are untargetable for 5 seconds after you use forms of crowd control on them. If this turns out to be too strong, you could add the 30% or what ever value decrease on the damage to untargetable champions. Either way, I think this would be a very cool mechanic that I would personally enjoy seeing implemented into the game at some point.