7) Max Mercury
Max Mercury has served as a mentor to everyone from Jay Garrick to Barry Allen and Wally West over the years, and it would actually be pretty surprising not to see this time travelling speedster show up in The Flash at some point in the near future. Whether him being revealed as the Man in the Iron Mask would be a satisfying pay off is hard to say, especially as it’s only going to be viewers with a good knowledge of the source material who recognize the name.
Still, introducing Max as a new mentor to Barry could be very interesting, especially as his comic book counterpart was at one time possessed by The Rival, another mysterious speedster who is essentially Jay’s own Reverse-Flash. With that in mind, Max could actually be revealed as the big bad of season three if that’s once again the case.