I'm discounting Mitchell and Khan right off the bat because they put blatant red herrings in the trailer to screw with people. I think the Dehner looking character is actually going to turn out to be Carol Marcus and they just included her in the trailer to make people who want to see Mitchell say "Hey, there's Dehner - it's gotta be Mitchell". They also included the hands touching glass scene in the Japanese trailer indicating it must be Khan because that happened in the last Khan movie!
The people who cut this trailer are playing on expectations and rumors and doing a brilliant job of it.
My guess is that it's actually a new character. Because why bother rebooting a universe if you're just going to tell the same stories? That said, if it is an existing character I think they'll go with Garth and change his story up significantly. He's a relatively unknown character and his episode, while memorable, wasn't the brilliant piece of work that Where No Man Has Gone Before, Space Seed, or TWOK were.