John Harrison is Sauron reborn. Seriously, look at the evidence. Both characters are played by Benedict Cumberbatch. We are looking at the first Middle-Earth Star Trek crossover, with endless sequels where the movie studios can even make more money off the fans than ever before.
Seriously though, I just hope the movie exceeds all the expectations and the villain is not a cookie-cutter "I must have my revenge and destroy everything" type of guy. That would be both cliche and boring.