If Cumberbatch is playing the role, then it makes more sense for him to be playing that character than Khan. Also, Mitchell requires nothing much in the way of backstory for the uninitiated, unlike Khan, who worked in Trek 2 (the real
Trek 2!) because his backstory had already been established.
However, I will remain skeptical because like a lot of folks here, I suspect all these villain rumors are part of a carefully orchestrated campaign to keep us wound up.
And in the end, it will be Q.