My theory, it will be the mirror universe.
Personally I would open with the Cardassians occupying Bajor with the Enterprise warping away because of the prime directive. Showing how Kirk has grown and give us fans a nod.
I hope and pray it will not involve the Borg or time travel.
How would the PD apply? If Bajorans were out star-skipping before we ever did, they're definitely not in the primitives category, except maybe with respect to their spirituality.
It would show that Kirk had grown, and Kira made references to how the Federation wouldn't lift a finger during the occupation. I just think it would make a cool teaser and give the fans a nod.
I know lots of folks know the original series better then me, but one think to consider is ST VI did really well. They could do the Klingon War and end it with the Khitomer Accords like in VI. Granted I think that would be an easy out, but going to Khan was an easy out too...