Constantine
This one is kind of a cheat because Constantine already had his shot at live-action TV stardom with a recent NBC adaption, but let’s be honest; Constantine never fit on NBC. He belongs in the Arrowverse so much that the actor who portrayed him, Matt Ryan, reprised his role for an episode of Arrow, a rare win for cross-network promoting. Even though he only had a small appearance, Ryan was charismatic and interesting enough to have fans pestering The CW to give him his own show.
Giving Constantine another shot at his own series would round out the Arrowverse with a magical element. Arrow deals with gritty vigilantism, The Flash and Supergirl are more straightforward heroics, the Legends infuse a bit of science fiction into the mix, and with Constantine, they could explore the more mystical side of things.
Make it happen CW!
Zatanna
If CW didn’t want to risk bringing back a character that already failed in fronting a show and would rather explore the mystical side of things but with a female-centric series, the obvious choice would be Zatanna. She’s a powerful magician with an interesting quirk of speaking backwards to cast her magic.
Her show would not only deal with the magic and mystical side of the DC Universe, possibly incorporating elements like the mystic tribe of sorcerers known as the Hidden Ones, or even set up Justice League Dark, but it would also be a really personal story as she struggles with escaping the shadow of her famous magician father. Heroes with daddy issues are CW’s bread and butter, so Zatanna would feel right at home.