"Different" timeline, but same universe. Plus, all the chief differences between the New 52 and the Pre-Flashpoint timelines are already being minimized or straight up erased so that the universe we're reading now is essentially the Pre-Flashpoint universe, but with a few key things missing still (which presumably Doomsday Clock has been aimed at explaining and/or correcting). That's pretty much what Rebirth has been doing for the past two years.
Except they already have been returning. For quite some time now. So, so many characters have had their origins and histories revert back to the way they were before Flashpoint. I mean...this very thread has been documenting that. Look at Martian Manhunter. Remember how in the New 52, he was apparently this guy who was on the outs with the League after getting into this huge brawl with them? Now, he's one of their most respected members and has this deep seated history with them...just like he did before Flashpoint.I doubt Pre-Flashpoint timeline (or memories) will return. It's seems more likely the characters will remember the "lost years" (which will be somewhat similar to Pre-Flaspoint timeline, but not equal) in the new timeline
Also, in regards to memories returning, again, they already have. Look at Iris West. She now remembers Pre-Flashpoint essentially verbatim to how it was, just like Wally. She remembers Wally's marriage and his kids, herself being married to Barry, Barry dying and Wally becoming the Flash, etc. I mean, that itself is in Flash #51.
So, all these memories and events have already been returning to the timeline.
I mean, I don't see that many Wildstorm characters running around in the current DCU, do you? They even relaunched the Wildstorm line in its own universe again.Not to mention that other things (like the fusion of three universes) happened in the Flashpoint.
But, again, he is no longer a replacement or rewrite for a pre-existing character. So, in essence, he is no different than just a new character introduced through any other means. I mean, Miles Morales started out in an entirely different universe (the ultimate universe) than the one he occupies now (the main Marvel 616 universe). He was only folded into the main Marvel continuity after Secret Wars. And yet, nobody is saying that the current Marvel timeline is a "different" Marvel timeline because of his presence.NuWally had mentioned in Flash 51 that he only exist, because the Flashpoint happened. Then, I guess he's one of the differences created by Manhattan.