10) Harley Quinn
This one might actually be familiar to many Arrowverse fans as much was made of a very Harley-like figure appearing in the Arrow season two episode titled “Suicide Squad.”
In case you missed it, Diggle and Lyla are arguing outside of an ARGUS cell, in which a woman with a Brooklyn accent calls them “puddin’” and says she can help them out as she’s a trained therapist. Obviously, it’s not too much of a stretch to realize who the woman is.
Sadly, Harley was barred from featuring any further due to the upcoming Suicide Squad movie (which was also the reason that the whole team was eventually killed off in the show). Still, the ramifications of Harley existing the Arrowverse are huge, as if she exists, surely the Joker does too, right?
And if the Joker exists… Well, you get the idea.