I have heard of this suggestion before, although it was quite a long time ago.
Behavior like that is rare enough, at least in my experience, that I don't think we need it.
I would prefer not to use the Greater Ignore, should it be implemented, but would prefer to use the Player Notes function.
I'd hate for a potential PvP foe to be made invisible by my enabling it. 
Oh yeah forgot about PVP 
Sig of Black Mercenary
What else do you want?
Now in a PvP area (Arena or Zone) I'm sure that the invisibility could be disabled, just leaving the /ignore upon his/her broadcast and such.
However, in a PvE area...
Let's say that you "Greater Ignore" a player named Bob. He sees that you are no longer reacting to him and goes streetsweeping. You no longer see him, so you go streetsweeping. You jump into a mob that he has already engaged (remember that you don't see him or any of his power effects?) and defeat the mob.
Sure, you might have seen the enemy's health dropping, but you might not have noticed, especially if his powers are holds and debuffs first, before damage.
He runs into another, nearby mob and starts his attack cycle. You jump in, not seeing him, and lay waste to the mob.
Does he now have a valid claim that you are a killstealer and harasser? He has no idea that you didn't actually see him, and to him, his power effects are perfectly obvious.
This scenario relies upon a lot of variables, but with enough instances of similar behavior and widespread use of Greater Ignore, it could eventually happen.
As an aside, I offer a bit of sympathy experience for whatever spurred your desire for this feature. Hopefully, you used /petition and also acted in a way that did not bring you down to thier level.
Unfortunately, I am not in favor of it being implemented.
Ever Ignore someone then have them follow you around and spam you with slap emotes and the like? What if there was a Greater Ignore that allowed you to not only ignore them but their character disappeared as if they never existed? I think it would be a great add-on considering alot of the jerks on this game.
This list would be seperate from ignore list and can be global or server-only.
Anyone else agree?
Sig of Black Mercenary
What else do you want?