6. Polaris
Polaris is one of the most fascinating characters in the X-Men universe, primarily because she is the daughter of Magneto, and she's also one of Xavier's original First Class line-up, despite ludicrously not being included in the film even though her boyfriend, Havok, was. If there's something First Class did extremely well, it was developing the back stories of its characters, and none better than Magneto, such that introducing his daughter - who isn't revealed as such at first - could be another way to add another dimension to Erik Lensherr and his family. Much like Magneto, Polaris is able of manipulating metal to her whim, and this could result in some fantastic climactic battle scenes if Polaris ends up facing off against her father in the ultimate war.