Unfortunately, they probably couldn't do a film that took place ONLY in the mirror universe. They'd have to work in some kind of crossover with the regular one. But even then, I guess they could make it work.
I don't see any way the Borg could be shoehorned into the next film, because even the Abramsverse Federation can't yet get all the way out to the Delta Quadrant. But there is always the opportunity to do a MU story.