90% certain it’s going to Gary Mitchell.
I’m thinking that it will be a storyline, along the lines, where the Enterprise either receives a distress signal or is ordered by Star Fleet to respond to a sip in distress, but arrives in time to see ship debris and a lone shuttlecraft, with a lone male occupant claiming to ba a John Harrison, who tells a lie as to how he managed to escape a ship that self destructed and the movie will go from there.
It’s possible. I think they would have to do a retelling of the Mitchell story, and not a continuation of it. So, I’m not sure where the vengeance angle comes in. If it were Mitchell wanting vengeance against Kirk for leaving him for dead on a hostile world (that plot sounds kinda familiar), then I could see it. But... then they would have to rehash the backstory for the benefit of the audience, which I don’t think they would want to do.
Plus, this is a young Kirk who probably hasn’t been commanding the ship for very long, so I doubt he has had time for his old adventures to come back to haunt him. So, if it was Mitchell back for revenge, they wouldn’t have the personal angle to work with, and it wouldn’t be as powerful emotionally.