From the trailer I leaning towards Khan, maybe Garth. But the fighting ability speaks to an genetically enhanced super soldier. Khan would have a bone to pick with earth. But again I am not ruling Garth out.
And in a way perhaps the Klingons find Khan and awaken him. I hate to quote something from Enterprise, but the Klingons had been fascinated with the augments. Perhaps they thought they could control him and were dead wrong. The fact that he is wearing a uniform of Starfleet is not evidence against either if you consider that Khan wore a broken Starfleet symbol around his neck in STII.
The one person I don't think it is now is Mitchell. One his story did not strike me as a revenge story. He was listed as killed in the line of duty as he did not choose to have those powers. The barrier did it too him. Where exactly is the revenge in that. Two he had god like powers, if he walked around tossing ships around I could see him being Mitchell but this guy seems like he is doing it the old fashion way.