When Jean Grey is possessed by the unstoppable Phoenix Force in next year's X-Men sequel, it's fair to say a few of her teammates are going to end up in a bad way. It's already been reported that she accidentally impales one of them, and it sounds like it'll be one of the major characters.
Either Magneto, Professor Xavier, Mystique or Beast is expected to die by her hand, and one of the series' big four is more likely to perish than the others. Jennifer Lawrence is said to have been reluctant to reprise her role as Mystique, making her the prime candidate for a fatal injury in X-Men: Dark Phoenix.
J-Law has admitted that she only returned to the series "for the fans" which sounds like code for "I'm working on an exit strategy". While killing off Magneto or Professor X would carry more weight, her status as a franchise regular should still make her death impactful and potentially tragic.
Mystique is unlikely to be the only casualty in Dark Phoenix, given the ultimate destruction the movie will unleash. It's possible minor characters will also snuff it, while Jean's ultimate fate post-possession is very much up in the air.