He's Khan, man. Think about it - would the general audience know who Gary Mitchell or Garth or Charlie Evans or Roger Korby was?
I still highly doubt that it's Khan and not so much from the trailer but rather from thinking from an Abrams point of view (perhaps an imaginary point of view- but still). If it is Khan then that means that Abrams- who usually delivers on his tasks- is coming to the second movie and thinking, "Let's do the second movie again, but this time without the invested relationship between Kirk and Khan that was in the original." So unless he's got some stellar twist of masterful writing with which to replace that investment, then he's going in knowing full well he's not only making an inferior version from the start- but one that is going to receive a lot of backlash from the fans. I know I'd be pretty disappointed.
Khan's plan in the original all revolved around the "white whale" of getting back at Kirk for what he did, and for his perception that Kirk was responsible for his wife's death and all that jazz. But here? What's going on? How does Khan present any type of threat? How can he think he's going to attack the Federation? Where's he getting his ship from? You know; in the original he used mind worms to control Chekov into setting up a trap for the Enterprise; what's he do this time? ear worms again? Another trap but without Chekov? All of these things seem to me to be really problematic. Genetically engineered supermen fighting against Kirk is one thing, but fighting the entire Federation? I just don't see this as being a feasible storyline. Not to mention the problem of Cumberbatch wearing the Starfleet uniform.
Okay but if it's Gary Mitchell, now it makes more sense. He's got super-ESPer powers which fade in and out. He is a "one man weapon of mass destruction"; he fits the bill to a tee. Some people object saying, "He's got no silver eyes in the trailer!" Well, yeah- if they showed him with silver eyes, the gigs pretty much up. So obviously they're not going to just give it all away in the teaser. It makes sense that he's wearing the Starfleet uniform, and that he can control others' minds and do these grand scale objectives.
I suspect what we'll see is him destroying some stuff, Kirk asking Pike, "Who is this madman?" and Pike retelling the story of WNMHGB and how he marooned Mitchell on the distant planet; we see the immediate effect of changing the timeline, causing Kirk not to be the one who was captain of the Enterprise during WNMHGB, and Pike making the wrong decision. Mitchell probably will kill Pike (perhaps the "WOK" mirror scene with Spock's hand and Pike's?) and then Kirk will have to stop him.
Also, I can't help but give a nod to the guy who noticed the marooning on Delta Vega appearing in both the first movie and WNMHGB, both at Spock's hand. Then there is also that the blonde looks incredibly similar to Elizabeth Dehner who is a psychiatrist (blue uniform) who looks to be getting mind-raped in the trailer (either that or Scotty has been into the Romulan Ale and forgot to put on his pants). Cumberbatch looks like a pretty good match for Mitchell as well. Plus, this is one of the first and all-time great Trek episodes and would be a good target for the reboot series.
http://www.pinkraygun.com/wp-content/uploads/2012/01/Star_Trek_silver-eyes.jpg
http://www.giantfreakinrobot.com/wp-content/uploads/2012/12/alice-eve-578x434.jpg
Then there is the poster which shows Cumberbatch in front of a bunch of destruction. Maybe it's Khan- but if I were a betting man (and I am) I'd put all my chips on Mitchell without blinking an eye.
http://bitcast-a-sm.bitgravity.com/slashfilm/wp/wp-content/images/stidintl-ru-post-t.jpg
[Hotlinked images converted to links. - M']
Is something you're not quite grasping.