9) Doctor Light
The Flash season two introduced a version of classic DC foe Doctor Light – here it was the Earth-2 doppelganger of Barry’s ex-girlfriend Linda Park. Though we haven’t seen him in action yet, there is actually another Doctor Light hanging around Earth-1.
In Arrow’s season two episode “The Man Under The Hood” (which also sounds like a reference to Batman’s Red Hood), future Flash characters Cisco and Caitlin aid Team Arrow against Deathstroke, with a light-based weapon. Cisco mentioned that it was created by their STAR Labs colleague Arthur Light, who was previously fired for “being a psycho.”
This is a reference to the original iteration of Doctor Light, who’s a long-running enemy of the Justice League. After clearly setting the character up here, it’s unknown why they decided to use another version of the villain in The Flash. Who knows, though? Maybe Psycho Dr. Light will pop up in the future?