Wolverine is the most popular character from X-Men and has several movie appearances but has not got any single project.
Apocalypse was born in the ancient Egyptian time and is one of the oldest mutants alive in the earth, he is one of the fan faves but hasn't got any limelight in movies.
Sunfire is the lower X-Men character but the main hero of Japan and is an important mutant as per the defenders perspective.
Magneto is one of the most important characters in the X-Men and the MCU really needs to think about his solo project.
Professor X also Known as Professor Xavier always has a leadership quality and defines the team mission.
Bishop is one of the mutants who came from an alternate future and took the teachings from Xavier and became an important character.
Cyclops is one of the best combatants and is compared to Captain America because of his skill and leadership.