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.