User blog comment:ZSomji16/The Flash (Season 4), Arrow (Season 6), Legends (Season 3), and Supergirl (Season 3) Villains and Big bads/@comment-24152851-20170327122115/@comment-24152851-20170328042628

I was thinking to keep him mystical, but also not having him too OP, they could have Trigon possess others like Lucifer from Supernatural. This would also be a good chance to have John Constantine guest star again.

Alright fair enough. Though I'd also add Brother Blood is another major Arrow villain who never had history with Green Arrow. Most of GA's major rogues have been used up so they keep borrowing from other hero's galleries.