1) The DC Universe Vs. Dr. Manhattan
As has become clear, we can only really guess what shape this event will take, but common sense says that it will ultimately boil down to the DC Universe taking on Dr. Manhattan.
For reasons unknown, he decided to steal five years of history from this world, but what does that mean? Did he do it for the extra power, or because he created the Multiverse and simply wanted to see what would happen? The changes to the timeline were previously blamed on Barry Allen’s actions during Flashpoint, but that no longer looks set to be the case, so some big reveals are definitely on the way.
Dr. Manhattan is damn near invincible, so how the heroes could combat him is hard to say, but they’re going to want answers for what he did to them and that has to lead to a fight. For all we know, the powerful being might even resurrect all of the Watchmen characters (including the Comedian) for the two sides to square off with each other in one epic battle.