The Justice League has had almost every conceivable hero in the DC universe as part of its ranks since the team debuted in the 1960s. Now that Wonder Woman is out and Justice League is on the way, these are three heroes that should join the team for its big-screen debut.
Green Lantern (Various)
Okay, so maybe this is cheating a bit, but this is also the most obvious choice to join the team. Green Lantern has been part of the team since its early days, offering another link to the more cosmic parts of the DC universe. The title of Green Lantern offers a variety of characters that could join the team in its efforts to save Earth.
There's, of course, Hal Jordan, the original Green Lantern. Hal got his first big screen treatment in the 2011 film starring Ryan Reynolds. Though it did not deliver what fans were hoping for, Hal is still a great character that should get a second chance. However, who said it necessarily has to be him since there are five other Green Lanterns from Earth that could join.
Another is John Stewart, a friend to Hal and a former soldier who translated pretty easily to the life of a Green Lantern. There's also Guy Gardner, a hot head, and Kyle Rayner, a creative mind, who both could provide unique dynamics in the group. Lastly, Simon Baz and Jessica Cruz, the newest Lanterns who are also the current ones in the Justice League comics.
Bringing in Green Lantern also offers some great story opportunities for future Justice League films. The biggest story is Blackest Night, which ran in the comics from 2009 to 2010. It pits Green Lantern and other heroes against the Black Lantern Corps., a collection of undead heroes and villains who rise from the dead thanks to black power rings. This serves two fold by telling a Lantern-centric story in the body of a Justice League film.
Another obvious choice is the Martian Manhunter. Hailing from Mars, the Manhunter is another League original, being one of the founding members of the original team. At first glance, J'onn J'onzz seems like a green-skinned clone of Superman, but there is more there than most may realize.
Superman never knew his own people or home planet but J'onn witnessed the destruction of his people and home first-hand. That has left him psychologically scarred to the point that he is vulnerable to fire. This also makes him, at glance, a cold and distant character, since his vulnerability is born of what is commonly known as PTSD.
This also provides villains that could step up to the League as well. Among the most common enemies are the White Martians, who would be the perfect alien invasion villain. These are aliens can shape-shift and infiltrate society before springing their attack. This would leave the team in a vulnerable position, with each member questioning if one of them is a White Martian.
Lastly, there's Shazam. Originally known as Captain Marvel, Earth's Mightiest Mortal is a character who could provide a welcome amount of levity to the team and universe.
Despite his power and appearance, Shazam is normally of Billy Batson, a young boy who was imbued with powers by an ancient wizard to defend Earth. This is another character that could provide a power match to Superman, as the two have fought at multiple points throughout the comics, mostly due to mind-control or other such circumstances. Whenever they fight, though, it has always been very pitched and even battles, with no hero able to get enough of an edge over the other.
He also provides a great villain who also happens to already be cast. Black Adam, who is going to be played by Dwayne "The Rock" Johnson, is an ancient being that rules over the isolated city of Kahndaq around Egypt. Adam was also imbued by the same wizard with the power to save his people, which he did before having tragedy strike him and his loved ones, causing him to snap and become something of a dictator. This provides another unique location such as Themyscira and Atlantis that further grows the universe.