There still seems to be confusion among editorial, and thus, creators, as to what the title and plot actually are, and it has changed several times.
The current plot might be comparable to Marvel’s Contest of Champions/Secret Wars from back in the day.
As of now, it is intended to be in-continuity and not an Elseworlds-type story.
It will feature some pre-52 characters along with New52 characters. So, apparently, the old pre-52 universe still exists somewhere, although how this is possible without invalidating Flashpoint is not clear.
As of right now, it has nothing to do with Anti-Monitor – many in editorial were surprised when Geoff Johns brought that character back. Some didn’t know about it until that issue was printed. This is not the first time they have been surprised by a plot point in his books…