Warner Bros. Television has done such a good job of introducing characters into the Arrowverse over five years that now it seems like anybody could be added and it would work. But each of the series have a different tone, meaning that certain characters would make more sense on certain series. Here is a look at fan-favorite character I think should be brought into the Arrowverse through The Flash Season 4: the mistress of magic herself, Zatanna.
This is a character that I can't believe isn't already in the Arrowverse for how popular she is, and I think she fits best in The Flash because the show is all about science. Part of what makes for good drama is putting a character into a situation so far outside of the norm that they are almost completely lost. Imagine doing that with Team Flash! Every one of them, from Barry Allen to Julian Albert, are scientists or logical thinkers that would refuse to believe in magic. What better drama could there be than having them deal with someone who does magic, and does it by speaking backwards? Having Zatanna guest star could open up the minds of the team and create for some fun scenes. And it would be another member of the Detroit edition of the Justice League brought into the Arrowverse.
Zatanna was created by Julius Schwartz, Gardener Fox, and Murphy Anderson and has such an iconic look that they'd have to stick with it. An actress like Nina Dobrev (The Vampire Diaries) would do a good job with the role and keep with the CW's tradition of hiring actors they've previously worked with. Maybe they'd even get a chance to work in Matt Ryan in a cameo as John Constantine.
The Flash Season 4 premieres in October.