Fox is currently conducting searches for a Native American actress to play Cheyenne mutant Moonstar and a South American to play Brazilian Sunspot, per THR.
Also yet to be cast in the teen-driven film: Kentucky-born Cannonball and an alien named Warlock.
Josh Boone of "The Fault in Our Stars" will direct and is co-writing the script. The movie is slated to start filming in July with an April 13, 2018 release date.
The X-Men-verse already tapped Williams' "Game of Thrones" sister, Sophie Turner (Sansa Stark), who played young Jean Grey "X-Men: Apocalypse."