5) Starman
Starman is a superhero mantle taken on by so many over the decades that he deserves to make an Arrowverse appearance based on that alone. Similar to Green Lantern, the character derives his power from a powerful Cosmic Rod. As the Starmen are often related to each other, a super-identity passed down through the generations could be very interesting to explore.
It would be the easiest thing in the world to introduce him, too. His hometown Opal City has been referenced numerous times on both Arrow and The Flash, as well as Supergirl. That’s two separate Earths with an Opal City – surely Starman exists in at least one?
Alternatively, Starman’s female successor Star Girl would be a good fit for the Arrowverse. DC head honcho Geoff Johns would presumably like to see her appear, as she is a character close to his heart – her likeness and personality were based off his sister Courtney.