8) Shade
First of all, we have to admit that Shade has previously appeared in one episode of The Flash. However, it was such a loose and underwhelming version of the character that it hardly counts as an adaptation of one of the most long-running and significant foes of the Scarlet Speedster in comic book history.
Dating back to 1941, Shade has fought Jay Garrick, Barry Allen and Wally West over the years. Originally a thief with the power to manipulate shadows, he was rebooted in the 1990s to be a much more morally-ambiguous character – a Victorian man granted immortality who became the mentor to Starman.
Despite his long and interesting history, the character made his live-action debut in season 3 of The Flash… and was portrayed as a complete non-entity. Just some dude who could turn into a shadow and was easily defeated by the STAR Labs gang. With the concept of the Multiverse at its disposal, though, a much more comics-appropriate version could easily show up in the future, as he could be said to be a doppelganger from another Earth. If Arrow can have two different versions of Count Vertigo, The Flash can definitely have two separate characters based on Shade.