The X-Men films have played around with Magneto and Mystique’s relationship a lot during the past 14 years. In the first three films they were close partners in the fight against humankind until Mystique lost her powers. In X-Men: First Class, Magneto taught Mystique a different mindset of how to view her mutation. In X-Men: Days of Future Past, the two of them found themselves as adversaries, although their endgames were remarkably similar. However, there’s one aspect of their chemistry that has yet to be fully delved into: love. Fortunately, the "will they/won’t they?" plot will be resolved soon...maybe.
According to THR, X-Men: Apocalypse will place the Magneto and Mystique romance "front and center." This is primarily being done to take advantage of Michael Fassbender and Jennifer Lawrence’s star power. Since First Class was released in 2011, both of their careers have skyrocketed, with Lawrence starring in films like The Hunger Games series and American Hustle, and Fassbender drawing attention for roles in 12 Years A Slave and Shame. Apocalypse is intended to be the final film of the First Class trilogy, which has explored the lives of young Charles Xavier, Erik Lensherr (Magneto), Raven Darkholme (Mystique) and Hank McCoy (Beast).
While First Class largely served as a Magneto origin story and Days of Future Past showed Charles embracing his role as Professor X, Apocalypse has been touting Mystique as its main focus. It’s already been teased that her and Beast’s relationship will also be touched upon, which means there may be a love triangle in the works. However, given their different ideologies, the future isn’t looking good between those blue-colored mutants. Days of Future Past saw Charles stopping Mystique from becoming a killer, while at the same time their brother/sister relationship changing when Charles realized he can’t keep trying to control her. When they meet up again in 1983, hopefully they’ll be on better terms.
Regarding Magneto and Mystique, nine years will have passed between Days of Future Past and Apocalypse, and it’s unclear whether the two of them saw each other during that time. If romance is being explored, Magneto may not have hard feelings about her shooting him and stopping his assassination plot. With the changes in the timeline, Mystique may not become the same brutal killer that Rebecca Romijn’s version of the character was. She still has her issues with humans, but she may not be as revolutionary as Magneto is.
All that said, the First Class trilogy has been teasing this romance with them kissing here and there, so it’s good this is finally being addressed, although I think these two are probably better off as friends/fellow soldiers with benefits.
