So with the information on 5G from Bleedingcool, we can kind of deduce the original plot from Bendis's Superman run and the Legion run.
According to BC, 5G Clark lost his powers and got it back, but it kept increasing and became a threat to Earth itself, so Batman sacrificed himself to stop Superman and lead to both of them dying.
So we go back to Bendis's Superman run, where he has been laying the pipes.
In his Superman run, Bendis indicates that Zod can see the future somehow, and in both future, House of El and House of Zod united to create New Krypton, but they were attacked in both visions, by the Legion of Superheroes?
In his Legion run, Bendis mentioned that Earth is destroyed, and Damian is a tyrant.
Based on the new information, these are sort of in line with the 5G plot, Damian is the leader of Leviathan, and Superman will destroy the Earth.
Jon basically went back in time to try and stop this, but he failed.
My theory: This actually was the original plot of Justice League vs Legion of Superheroes. I think the Great Darkness that Dreamgirl foresaw was actually Superman, so they went back in time to stop Superman, in order to stop Earth from blowing up. At this point, House of El and House of Zod is on New Krypton, and the Legions will attack them, so that explains the visions by Zod. And this will lead to an actual Justice League vs Legion battle, where Legion is trying to stop(kill?) Superman and the Justice League trying to stop the Legion, which will cause both the Justice League and the Legion being disbanded. Similar situation to Civil War 2, which is also written by Bendis.
So Legion 2019, 5G 2020, Justice League vs Legion 2021.
But obviously the plans changed, and that story became the current Justice League vs Legion story.