Technically, they previously had two champions like this: old Xerath and Kog'maw.
Old Xerath's ult range was HUGE, had a large hitbox, did tons of damage, and didn't have an ult range indicator. Combined with his Q, W, and R, he was technically a Turret with insane range. If you don't know what I'm talking about, look it up.
Kog'maw technically has the mortar aspect down, but he has to be within a reasonable range to deal damage.
From what you are proposing though:
"... slightly larger than 2 lanes."
"... hide and go seek to find the person who is incredoubly annoying, and kill them."
These two aspects bother me. You're stating the champion can freely harass from more than halfway across the map. There's no counterplay to this.
Oh wait, I was wrong. Xerath was like this a few patches ago; not sure if you fought it, but you've probably heard of it. There was an exploit (hack) that allowed Xerath to hit everything globally with his Q from his base. It was perhaps one of the most annoying and frustrating things to fight against. However, what you're proposing is a slightly less toxic version of this...
Although if you are proposing a champion like this, there needs to be a few restrictions. One, the champion needs to be incredibly immobile. Two, the enemies must get vision of that champion or the skill if it were to use such an attack from far range. Three, the attacks must be visually telegraphed and have a delay to enable counterplay. Four, there must be a significant delay between barrages or have a relatively high cost to prevent spamming.
