Guys haven´t you figured it out yet? It´s not Khan. It´s not Gary Mitchell. And it´s definitely not Garth of Izar. The baddie is Captain Ronald Tracey from The Omega Glory. First of all, Khan isn´t shooting at klingons in the trailer, but Kohms. So it´s a reimagining of that episode. At the beginning of the movie, Spock and Tracy go to Omega IV to stop the war between Yangs and Kohms. But Spock loses contact with Tracey, and has to beam back up to the Enterprise alone. But Tracey isn´t dead and he hires a gang of Kohms to join him on his mad quest to conquer Earth. That´s why he says "I have returned" and then the phrase "You thought your world was safe" is really about that Starfleet has forgotten its roots, and isn´t following its constitution. The scene with lots of people in San Fransisco is really a political meeting where future leftists have gathered to protest and join Tracey´s movement. But luckily Kirk and Spock save lots of Yangs before Omega IV explodes (the volcano scenes), and go back to Earth with them. The only problem is that the Yangs can´t fly a starship so they crash the ship in the Bay area. And you know what the ending will be...yes, Kirk reading the constitution of the Federation, which is the same as the US constitution. So the movie is a big metaphor about the current state of civil rights.