Because the other threads have already hit auto-sage or are about to go there.

SPOILER TAGGED IN THE OP, BUT Y'ALL ALREADY KNOW WHAT'S UP PROBABLY.

So the major question of the day seems to be to what extent the influence of Watchmen and Dr. Manhattan have had on the wider DC Universe. Has he been able to single-handedly manipulate events, or is he bound by other forces like the Empty Hand? Or possibly an extension of such given that Watchmen is often credited with driving superhero comics in a much darker, grimmer direction in its wake.

There's also the question of how this relates to Earth-4, specifically with Pax Americana specifically riffing on many of the same themes and styles of Watchmen, despite Earth-4 Pre-Flashpoint appearing to be closer to its original Charlton comics counterpart.

Given that we are talking about a Johns work, I wouldn't consider anything we'd find in Rebirth to be drastically changing our understanding of the structure of the DCU Multiverse and cosmology, and if anything, with the addition of previously-blocked legacies and continuity and history, this might represent another continuation of what we saw at the conclusion of Multiversity, with the DCU truly becoming an Omniverse rather than being bound to ONLY 52 Earths.

Commence wild conspiracy theories, ladies and gentlemen.