It's not Garth of Izar.
It's not Gary Mitchell.
It's not Harry Mudd, Trelane, Q or the Third Klingon From The Left.
They're deliberately building up the mystery about Cumberbatch's character. This means one of only two things:
- He is someone the movie audience will recognize; or
- Revealing information about him will disclose too much information about the plot for other reasons, which means he's directly connected to someone or some event that the movie going audience will recognize.
So, the movie turns on someone or something already established and well-known to the general public. Otherwise the reveal is the equivalent of declaring that John Harrison is really John Doe.
That rules out everyone but Khan or the Borg, really.*
It's not the Borg.
*(or Luke's father)