The Wrap reports in their story of new Black Mirror episodes from Netflix that the series lead, Gugu Mbatha-Raw is expected to join the cast of Star Wars: Episode VIII. However, The Hollywood Reporter somewhat contradicts their report, adding that Mbatha-Raw was indeed in discussions to join the cast but is no longer in the running. While you could typically take The Wrap's reports as gospel, the contradiction from The Hollywood Reporter makes us advice that you consider this a rumor for the time being.
Back in September, Mbatha-Raw appeared on a shortlist alongside fellow actresses Tatiana Maslany, Gina Rodriguez and Olivia Cooke. (Again, The Wrap would have us believe that Mbatha-Raw has secured the role while THR reports that she's no longer in the running.)
If the role in question is for an alien, Mbatha-Raw's potential casting doesn't give us any clues. However, if spoilers from The Force Awakens are to be believed, there's a very likely identity for her character if the role is for a human.
Star Wars: Episode VII opens in theaters on December 18th. Director J.J. Abrams will relinquish the director's chair to Rian Johnson. Johnson will write the script for Episode VIII and IX but relinquish the director's chair to Colin Trevorrow. Benicio del Toro is confirmed to play the villain in Episode VIII.