By far the most likely character to meet their untimely end in the next X-Men movie is Mystique (Jennifer Lawrence), largely due to Lawrence basically sleepwalking her way through the role in X-Men: Apocalypse and clearly having little interest in the character anymore beyond a paycheck.
With Lawrence generally tackling more daring roles these days as evidenced by mother! and Red Sparrow, she can collect one more easy payday here before dying in the third act at the hands of either Jessica Chastain's shape-shifting villain or perhaps even the Dark Phoenix (Sophie Turner) herself.
Considering that Lawrence's interpretation of Mystique is generally accepted to be inferior to Rebecca Romijn's earlier portrayal, there won't be many wiping tears away at her death, but at least it can serve a grander thematic purpose by pushing the younger X-Men she's mentored to finish the battle.
If she somehow survives, it'd be shocking, honestly.