Booster Gold
There can’t be a DC character more requested by fans to appear in Legends of Tomorrow than Booster Gold. As the DC universe’s premier time-travelling hero, it just makes too much sense for the character to appear on the show for them to waste the opportunity.
Real name Michael Jon Carter, Booster Gold travels from his native 25th century to the present in order to seek glory as a superhero, with the aid of his future tech. Fans love him thanks to his brilliant character arc: he begins as a selfish glory hound but he later grows into a true hero and firm member of the Justice League. And, wait for it, he’s also the father of Rip Hunter. There’s a ready-made connection to the show right there!
Of course, what’s probably stopping the Legends team from using Booster is the fact that DC are also working on a movie starring the character. However, it just so happens that Arrowverse exec producer Greg Berlanti is working on it. Surely he can pull some strings to get Booster on Legends of Tomorrow, as well, right?