I'm betting on Arne Darvin, from what I've heard, they didn't use him in "The Truth About Tribbles", that leaves him open for an appearance in this movie. Darvin would be the perfect villain to use. He's established in canon, has only been used twice, and he's still enough of a mystery that they can do just about anything with him. Maybe this time his eco-terrorism will be targeted at New Vulcan as opposed to Sherman's Planet. Hell, for all we know New Vulcan might be Sherman's Planet. Regardless, the Federation had to have been shaken up by the destuction of Vulcan, and another power is bound to take advantage of it. Why not the Klingons? I've been waiting a long time for them to be bad again.