Rumors are 20th Century Fox wants to adapt the Marvel comics "New Mutants." "New Mutants" is an extension of X-Men, Like its parent title, It featured a large ensemble cast, including the introduction of Deadpool. With the end of the first series, the characters were relaunched as X-Force in a new series. Sunspot was one of the main characters, he was played by Adan Canto in X-Men: Future Days of Past.
Wolfsbane
A female werewolf. Pretty much can be played by any actress but I think someone like Shailene Woodley like Willow Shields of Hunger Games can handle it. Rahne was a shy emotionally repressed girl. She's 16 but I guess she will be of age once the movie is made.
Cypher
Doug Ramsey is a mutant with the ability to easily understand any language, whether verbal or written. He later forms a relationship with Wolfsbane. Even one form of Legion had a crush on him. I was thinking of Ross Lynch from Austin & Ally.
Cannonball
Sam Guthrie is a mutant who uses the ability to fly at jet speeds while encased in an impenetrable force field. Austin Butler who has been in Switched At Birth and The Shannara Chronicles
Karma
Karma is a mutant endowed with the ability to seize control of another's mind, though she has sometimes been depicted with other more extensive psionic abilities. Maybe Ardan Cho from Teen Wolf. She can make an audience feel.
Mirage
Danielle Moonstar originally possessed the psionic/psychic ability to telepathically create illusions of her opponents' fears or wishes. She later developed a wide range of psionic and energy manipulation powers. Since she is Native-American and there aren't many popular actrsses that fit that discription, I imagine this would be a newcomer.
Chamber
Was part of Generation X and New Warriors but I like this character so much I would squeeze him into New Mutants to replace Sunspot. Jonothon Starsmore possessed the ability to cast energy blasts from his chest. He had limited control of his power and destroyed much of his chest and lower face when his powers emerged. They did a Generation X TV Movie in the 90's and he was not in it because the special effets were not up to par back then. The special effects could definitely be done now. Thomas Barbusca od Preacher and Middle School is young but he again will be of age by the time the movie is made.