Ultimate Ability Idea

Seeing a lot of hype about the new Ashe skin kinda threw me back to Diablo 3'd Demon Hunter class, and gave me an idea for a unique ability in League.

So on some form of new, preferably ranged champion. I'd like the ultimate to follow a similar mechanic of the Demon Hunter's Rain of Vengeance skill. You target an area x-units away from you (with a minimum cast range so you'll have to be at a distance to use it.

Upon activating the ability, you root yourself for 3 seconds, zooming out similar to Jhin's or Xerath's ultimate. You then begin firing a volley of projectiles in an area that follows your mouse for UP TO the next 5 seconds, dealing less damage over time. At the end of the channel, you then fire an additional projectile that deals damage and slows all units hit by 30% for up to 3 seconds, depending on the amount of time channeled.

Some iteration of locking yourself in place and throwing a huge aoe spell that slowly crawls towards the cursor would be AMAZING imho. Any thoughts? :0