3) Superman
Yup, the Man of Steel himself just might be in the Arrowverse somewhere. He obviously exists in sister show Supergirl and, though it doesn’t take place on Earth-1, that could have major repercussions. The Flash season two showed us that each Earth has doppelgangers of the same person, which means that there could be another Superman on Earth-1.
Also, in The Flash season one, supercomputer from the future Gideon said that Barry Allen was “a founding member of the—” before being cut off. This was surely a reference to the Justice League. And we all know that you can’t have a JLA without Superman!
The biggest hint to his existence occurs, however, in a juicy line of Rip Hunter’s in Legend’s third episode. “I’ve seen men of steel die and dark Knights fall,” he says, seemingly confirming that Superman in the Arrowverse. As well as a certain other hero…