I'm still thinking Cumberbatch is playing Khan and Peter Weller is playing an advanced age Gary Seven who seeks out Kirk to capture Khan.
I can definitely see Weller portraying Gary Seven, but I really, really
hope that Cumberbund isn't playing Khan. Personally, I think Urban really did let the cat out of the bag by accident, hence his back-peddling now. Based on what I've seen of BC's acting thus far, I think Mitchell would be a good match for him, especially a Mitchell who's ESP has turned him into a super-human of some sort. Plus, I think Mitchell as the 'bad guy' is more interesting than doing Khan again, simply because of his friendship and history with Kirk.
My guess as to what role Weller is really playing would be an operative of Section 31. I could be wrong, but I think it would fit with the potential Mitchell storyline.