When Jessica Chastain was cast in X-Men: Dark Phoenix, many fans immediately hoped she would be Lilandra, the Queen of the Shi'ar Empire. Chastain has since refuted those claims, though she hasn't revealed who her character is and if she's from the comics. Entertainment Weekly finally gave some new details about the film and Chastain's character but has not revealed the name for some reason.
EW has described as "an otherworldly shapeshifter who comes into contact with Phoenix." To say the details are vague would be an understatement, though we at least know she won't be from Earth. An image of Chastain's character was also released but it doesn't reveal much about her, though we know she'll have blonde hair.
Considering how The Phoenix has interacted with the cosmic side of Marvel, it's hard to guess who this character actually is. Lilandra doesn't seem to be an option since the character is an "otherworldly shapeshifter," something the Shi'ar Queen isn't. It could be an all-new character made for the film, which could be a bit divisive but isn't the worst decision ever made.
X-Men: Dark Phoenix is currently slated for a November 2, 2018, release.