At first, I was convinced he was Khan. Not my first choice to play him, but he made a lot of sense until more information came about.
Now, I'm fairly certain that he is, in fact, Garth of Izar
as a couple of others believe. Garth was a member of Starfleet, a contemporary of Pike, a masterful tactician, and a looneybird bent on genocide and universal domination who was eventually locked away. The fact that he learned how to change his shape fits in really well with having an assumed name (which only makes sense since there wouldn't have been any reason to keep it a secret for so long if he was in fact a new character), that he was an explorer, that he is a recognizable character from TOS, and that revealing one of his pseudonyms is a way to give people what they want without letting the actual secret out all fits.
At least far more than Gary Mitchell or the others listed on the poll anyway. The fact that Cumberpatch bears a (very, very slight) resemblance to him is just icing on the cake.