Are the early bets already open? I put my money on the Dominion.
I love The Dominion, but I don't think introducing them in XIII would be feasible. There was considerable world building in evolving The Dominion into the popular antagonist they became. I'd love to see them reinterpreted on the small screen, but setting them up for the big screen strikes me as too cumbersome.
That's where I think The Borg have it over The Dominion. Their premise is pretty straight forward and wouldn't require any lengthy exposition. Another consideration is average Joe Blow will be more familiar with The Borg (star trek, cyborg robots, cubes) than The Dominion (???).
Whether they can devise a story they want to tell is another story. If a certain character in XII is still present in XIII, they might have something else planned. But, for the sake of argument, we'll say they're considering The Borg. What story would you tell? What changes would you make? Standard invasion plot? Enterprise infiltrates Borg space? Do The Borg come in peace as a seemingly more enlightened collective?
The more I think about it the more I think... No. probably not.