The character did technically appear in season 1, although he was simply referred to as Mr Blank. He was an assassin who appeared in Home Invasion, and was killed by Oliver in the same episode. The characters creator Kevin Smith said that Onomatopoeia would not translate well to the screen, hence his transformation into Mr Blank, but there is still a chance the writers could decide to include him, and plenty of demand for it as well. He only targets non-powered superheroes, which makes Green Arrow an obvious enemy, has a secret identity, and he gets his name from his habit of repeating sounds he hears around him. It may have been said he wouldnt translate well, but he certainly sounds like a character the show could have a lot of fun with if they do introduce him properly.