Now that Matthew Vaughn is officially onboard to direct X-Men: First Class 2 (not the actual title), he'll be working steadily alongside writer/producer Simon Kinberg to decide where the sequel goes next, in terms of plot, and which returning characters will be more, less, or equally emphasized as they were in First Class.
James McAvoy previously suggested that First Class 2 may focus heavily on the transformative process which his X-Men universe alter ego (Charles Xavier) undergoes, in the aftermath of the crippling injuries he sustained in the first film. It's possible Xavier's character arc in the sequel will somewhat mirror Erik Lehnsherr's (Michael Fassbender) in First Class, partially for that reason.
Kinberg has spoken with MTV about the tentative plan for First Class 2, saying that the film should do "some things unexpected" with both Xavier/Professor X and Lehnsherr/Magneto's personal storylines. However, he is currently expecting the sequel to follow in First Class' foosteps - by being especially Magneto-centric.
Here (to quote) is what Kinberg offered MTV on that matter:
"That’s what was so cool about the first one and what we want to continue. Magneto becomes the villain ultimately of the franchise but he’s a much more complicated character as a young man. He’s someone you sympathize with, you care about you root for even though might not necessarily agree with his methods, you understand his philosophy."
With regards to whether or not First Class 2 will conclude with Magneto developing into more of an outright villain (as opposed to a more morally-dubious figure), Kinberg said:
"We’re still figuring that out. I think the truth is ultimately that because Michael [Fassbender] is such an interesting actor, he’ll never be full-on anything. He’ll have a villainous side and he’ll have a sympathetic human side you’ll be able to relate but you’ll also be afraid of him. He becomes Magneto, he says 'Call me Magneto' at the end of the first movie."
Bear in mind, it's best to not interpret Kinberg's comments to mean that First Class 2 won't heavily explore the bumps and roadblocks that Xavier encounters on his personal journey towards becoming the calm and collected Professor X - or that the sequel will feel even more like "The Magneto Show" than First Class did (at times). The First Class followup is just barely beginning to take shape, so nothing is set in stone right now.
As to which new X-Men characters or classic comic book story elements will be incorporated into First Class 2 - for now, that remains a periphery issue. Kinberg and Vaughn (and whoever else ends up working on the film's script) first have to pick up the narrative threads left dangling at the end of First Class and determine where to take them next, before turning their attention to new material.
X-Men: First Class 2 is tentatively expected to reach theaters by Summer 2014.