Cullen Bunn is laughing at us!(He probably isn't). He said he hasn't seen anyone guess correctly who the secret member of his team is yet. Here is a thread to discuss options. So far all we know is that this person was an X-Man.
Here are some ideas I have.
Mach 2 - Nomi Blume - Teenage mistress of magnetism, the ultimate universe's Magneto to Kitty Pryde's Professor X. If Miles Morales and Ganke survived the destruction of the Ultimate Universe why couldn't Nomi? Could be a great protege for Magneto. She was an X-Man in the ultimate universe.
Moira MacTaggert - Technically she is dead but the Terrigen Mists storyline is the best reason to go dig Moira up. No one including someone blue and furry who likes genocide knows more about the mutant race than Moira. She cured the Legacy Virus. She also has lots of drama with Magneto. Moira was technically an X-Man due to her stint on the Muir Island X-Men.
Both probably seem a bit left field but given that no one has apparently guessed it I don't think it's going to be someone like Polaris, Rachel Summers or Sage etc.