Onomatopoeia (let's call him Ono for short) is a Green Arrow villain created by occasional Flash director Kevin Smith and Phil Hester, and was scheduled to appear on Arrow back in 2013, before former executive producer Andrew Kreisberg changed his mind. This villain mainly targets non-superpowered heroes, such as Green Arrow and his team of vigilantes.
Yeah, the name might be silly, but he is one of Oliver Queen's deadliest foes. In the comics he had been shot with multiple arrows and was still able to move freely without showing any form of pain, this would make us believe he is a metahuman. Something that could bring him to the show is that he views Team Arrow as a challenge and could hunt them like animals. Arrow is at it's best when the characters are being chased, and this would pose no exception.
Ono is a character that makes more sense in the comics, but the writers for the show are usually great at adaption and finding ways to make them work. A season is only as good as it's villain, and hopefully, Arrow would be able to nail this one. In the six seasons of Arrow, they have yet to have a meta as the main antagonist of the season, and it would pose a new element to the show that it desperately needs.