Matthew Vaughn's X-Men: First Class was widely panned by fanboys and casual movie goers alike while it was in development. Much to everyone's surprise, the movie was quite good and performed very well at the box office. Naturally, Fox Studios is thinking franchise and the actors have been discussing the planned trilogy all summer long. Discussing it with Collider and Hero Complex, James McAvoy stated:
“Ideally you have a story arc that lasts three movies and at the end of the third it leaves you ready for the story in X-Men 1. It would be nice to explore things in two more films. I’ve lots of ideas. I know Michael[Fassbender] and I are very much on the same page all the time and we’ll be weighing in to protect that relationship between the two characters. The next movie, if there is one, shouldn’t just start off with them being pals again, but I think it also shouldn’t be like the first movies only set in the 1960s. If we get another, let’s not just make Magneto the bad guy; of course he’s a bad-ass and of course he has a whole different ethos, but making a movie that is black-and-white is going to lose the thing that we have in this one.”
Judging by, the critics review of X-Men: First Class, I would say that a sequel is well warranted.
Furthermore, with a budget of $150M and a box office revenue of $348,728,454, I'd say a sequel is all but guaranteed. Vaughn chimed in with the following on a sequel:
“I’ve got some ideas for the opening for the next film. I thought it would be fun to open with the Kennedy Assassination, and we reveal that the magic bullet was controlled by Magneto. That would explain the physics of it, and we see that he’s pissed off because Kennedy took all the credit for saving the world and mutants weren’t even mentioned. And we could go from there, and I’ve got some fun ideas about what other mutants to bring in. I don’t want to tempt fate, though. If the film’s a hit, of course I’d be interested. I really enjoyed making it. We will only have one more new character. I won’t say who he…I won’t say who he or she is! But we will only be bringing one more new character in. Because, I think, as Professor X is in a wheelchair, Magneto needs to have a nemesis he can fight with. Someone that will be his equal. I know who it is. It would be nice if I could say something, but I can’t, mate!”
With that said, who's the one character to be introduced that can be an equal to Magneto? Based on Vaughn's statement we can dismiss any notions of introducing any of the original team members. And if they want to keep Fassbender's Magneto "grey" in terms of character tone, the new character must be a villain [Fun Fact: Stan Lee originally planned for Magneto to be brothers with Professor X.] Personally, I would like to see Mr. Sinister introduced. He's such a cool character that has deep ties to the X-Men mythology and would be a visual treat displayed on the big screen. Who do you guys think could portray Sinister on the big screen?
Mr. Sinister:Mister Sinister is a fictional character that appears in comic books published by Marvel Comics. The character first appeared in Uncanny X-Men #221 (Sept. 1987) and was created by writer Chris Claremont and artist Marc Silvestri. Mister Sinister is a genetically altered human with superhuman physical and mental abilities. The character is telepathic, able to manipulate the minds of others in various ways, and is capable of energy projection through telekinesis. Using genetic material from the mutant Courier, Mister Sinister gained complete control of his body at the cellular level, allowing him to shape-shift, regenerate and be virtually immune to injuries. Mister Sinister is also a scientific genius, with expertise in the fields of biology, genetics, cloning, physics and engineering.
Fabian Cortez: Fabian Cortez is a fictional character, a mutant comic book supervillain from the Marvel Comics universe and an adversary of the X-Men. Created by writer Chris Claremont and writer/illustrator Jim Lee, he first appeared in X-Men #1 (October 1991). Cortez possesses the ability to bolster another mutant’s power. He could enhance a mutant’s abilities to occasionally dangerous limits beyond their control (temporarily powering them up at the expense of burning out their bodies). In X-Men #1 (1991), while onboard Asteroid M, Magneto returned from a fight with the X-Men having suffered deep lacerations to his lower abdomen from Wolverine's claws. Cortez "healed" Magneto (as he said he had also done with his sister after an earlier battle) and temporarily brought him back to full health. This turned out to be "fake" healing in that Magneto needed to regularly be "healed" by Cortez to stay uninjured.
Mesmero: Mesmero (Vincent (last name unrevealed)) is a fictional mutant, a comic book supervillain in the Marvel Comics universe. The character first appeared in [Uncanny] X-Men #49. Mesmero was a mutant who has the psionic ability to mentally control the minds of others. Through a combination of careful planning and skill, he could also manipulate the minds of telepaths; though he had to take extreme care, as a telepath was much more prone to realizing/breaking his machinations than a non-psi. He could hypnotize people into doing what he wanted them to do, and alter their minds with false personalities and memories. He could also make them see him as a different person.
Apocalypse: Apocalypse is a fictional character who is an ancient mutant that appears in comic books published by Marvel Comics. The character first appeared in X-Factor #5 (June, 1986), created by writer Louise Simonson and designed by artist Walter Simonson. Apocalypse is portrayed as an immensely powerful mutant who was capable of going toe-to-toe with the Avengers and Cable at the same time. Even when severely weakened, Apocalypse was able to defeat an assembly of X-Men.
Apocalypse possesses several distinct superhuman powers, some of which are owed to his mutant genetics and others due to technological improvements he has made on himself. After living several centuries, he further augmented himself by Celestial technology, though it is unclear to what degree these enhancements granted him any additional powers beyond what he already possessed. His powers appear to change according to his need to rest his long-lived body and/or the creative team's desires.
Apocalypse was initially portrayed as a shape-shifter on a molecular level, who also had the ability to teleport himself and others over unknown distances. His power set has been greatly expanded since then, and he has displayed immense strength, near invulnerability, rapid regeneration, energy absorption, and projection. He is resistant to telepathy and psychic attack, and may have limited telepathic and telekinetic capabilities of his own.
Apocalypse has complete control over the molecular structure of his body — he can alter his form as it suits him; examples include allowing his body to become extremely malleable, changing his density, and even increasing his size and strength by taking on additional mass from an extra-dimensional source. Apocalypse can stretch or contort his limbs and body into a multitude of shapes and sizes, with elasticity and flexibility far beyond the natural human limits. He can open holes through his body to avoid attacks. He is able to form various weapons (e.g. blades, hammers, guns, or shields) with parts of his body, including wings or jets to enable flight. He is also able to adapt his body to apparently any disease or environment. On one occasion, he has shown the ability to phase his hand through another object/person. Through his shape-shifting ability, Apocalypse can give himself virtually any physical superhuman power and disguise himself in a vast variety of forms.
Cable/Bishop: Time traveling mutants that kick ass! That is all.