None of the above, although it involves Team Rocket.
Have the best of the Rocket fighting force show up at the Spear Pillar to do battle with the Galactics. Bear in mind J will probably factor into it as well. So there'll be a lot of temporary "unholy alliances" going on.
So the ultimate battle I have in mind for DP is Mars, Saturn, and Jupiter vs Domino, Tyson, Brock, and Dawn, while Ash, Giovanni, J, and Cynthia do battle with Cyrus who's absorbed the powers of Dialga and Palkia (lest you think that last bit sounds like a bad fanfic, he *does* mention in the game that he "makes the power of pokemon his own").
Then of course the battle would lessen to J vs Cynthia because she's just going to cause more trouble. Unless they have J ultimately get away like Brodie.