User blog comment:Тостер/Prometheus: Unusual Suspects/@comment-24152851-20161112070857

One suspect I think deserves a mention is Slade Wilson.

He is one of the characters who holds the biggest grudges against Oliver, yet has an uncertain fate. He never got to complete his revenge. He is on par with Oliver's skill (without mirakuru), having trained Ollie himself. Plus, he is the only character aside from Malcolm Merlyn to serve as an "anti-Arrow" in the past, having a very similar origin, but having opposite intentions. Also, his return in season 5 was teased at on social media.

I doubt it's, Malcolm since he's confirmed to be in the Legion of Doom, so he's probably too busy annoying the Legends than to play double duty and terrorize Star City as well.