I don't know what this big event is going to entail, but here are a few things I think will come into play.
Daxam - There was just a whole annual dedicated to Krypton's sister race and it's history. This paired with Mon-El's presence in SUPERMAN leads me to believe the Daxamites will be major players.
Brainiac - Yeah, this guy's story is still building.
The Legion - We know the missing Legionnaires are in the 21st century. We've seen Tellus, Sensor Girl, and, of course, Starman(boy). We've also just been told that
I think in the end Kandor's going to end up somewhere out of Earth's solar system.
Superman and Supergirl (and Krypto) will be the only powered Kryptonians left on Earth.
And someone will will told to "Kneel before ZOD!"