If they bring back enough of a pre-
Flashpoint timeline that Diana is not the daughter of Zeus, and Donna is not a weaponized construct created for malicious purposes, that would be the biggest gift DC has given me since the days of
Darkest Night and
Brightest Day.
Really, I would be perfectly comfortable if they took the opportunity of
Doomsday Clock to include, and exclude, various elements from various different continuities, and add some new things too,
if (a) the resulting main timeline looks - from the inside, and the viewpoint and memories and histories of the characters within it - organic rather than an obvious, non-coherent patchwork; and (b) they take some time to tell us what the history of this new timeline and its characters actually is, rather than leaving everyone (not just the readers, but the writers and the editors too) to guess, and wind up with conflicting answers.
For example, right now the story of the Titans (particularly the O5) is: they were sidekicks to the Justice League (except maybe Donna), formed a team together, and became best friends. Then, after an incident with Mr. Twister, they and their mentors and everybody in the world forgot that they ever got together as a team (and, in Garth's case, that he was ever Aqualad at all). They then spent a number of years going on with their lives on the basis of deleted and changed and fake memories, barely knowing each other, not treating each other as friends, and not remembering their many experiences together. Then they and everybody else (I
think everybody else) got their memories back, and they've come together as a team and friends again - but the courses of their lives were still, none the less, tremendously distorted and diminished (on an emotional level) by living those years based on fake memories.
Personally, I think this is a
terrible backstory to give to a bunch of good characters. Especially if you have no interest in really examining the impact of all that weirdness of character development, which TPTB don't. (The current Titans are written more like they just got back from a month of separate vacations, and haven't seen each other for a while, as opposed to people whose key personal relationships were ripped from their minds for years.) And the only reason it's there - the only purpose it has - is an attempt to "mesh" what we saw in The New 52 (the O5 Titans barely know each other; Garth was put on an elite team to hunt down and battle Aquaman, who he didn't even know personally; and so on) with "look, we've got our old Titans back!" And it doesn't even really do a good job of that, if you think about it.
Well, there's really no reason to do that now. Just admit that the New 52 stories of the O5 Titans (at least those stories that touched on, or avoided, their relationships with each other) - even if some of those stories were good - are no longer (as originally written) part of the current timeline. Maybe they happened in some other timeline, maybe they didn't. But in the post-
Doomsday Clock timeline, the O5 met as kids, formed a team, became best friends, and maintained those relationships to this very day. No world-wide memory wipes (utterly unnecessary). Garth never hunted down and fought Aquaman, at least not the way it was written. If you want to get rid of the
consequences of those New 52 stories, and Dr. Manhattan is playing with time anyway, then just get rid of those stories. (From the new timeline, at least.)
And if there are stories from The New 52 that you want to keep (the Court of Owls, for some readers), then keep them - but knit them into the new timeline so that they make sense there, and the timeline is coherent. (This isn't even difficult with the Court of Owls, since Batman's history, as opposed to the Titans history, was not so affected by The New 52.)
A new thing they can include? Give Donna Troy an origin in which she is a human girl from Man's World who was raised for a time on Themyscira, adopted by Hippolyta, and was Diana's loving adoptive sister. (This was basically her pre-
Crisis on Infinite Earths origin after she was a member of the Teen Titans.) And add into the history that, when she was Wonder Girl, she sometimes worked alongside Wonder Woman as her sidekick, like Batman & Robin or Flash & Kid Flash. Actually show us some flashback stories like that. Flesh out their early relationship. You might even show us where and with whom she lived on Man's World in those Wonder Girl days.
(Bizarrely, she never had that. After she was given the Diana-Rescued-Donna-from-A-Fire origin -which has small problems but can be fixed - the relationship between her and Diana was
implied. But, for example, she rarely if ever showed up as Wonder Girl in Wonder Woman's own comic. Meanwhile, Aqualad was still showing up in Aquaman stories, and Speedy in Green Arrow stories. Donna never got that visibility, that solidity. Which is why, after
CoIE moved Diana's origin to be years later than Donna's, it was so easy to give Donna a new origin in which she had no connection to Diana to speak of, and still act like nothing much had changed for her. I thought losing that relationships was a shame, and a big mistake. Well, now they have the opportunity to fix it, if they want to.)
I don't really like careless retcons every few months, done without much regard for their consequences. But Rebirth/Reborn was a
major retcon, with, of necessity, ripple effects that affected all the heroes in the DCU. (For example, the Superman/Wonder Woman love affair was
gone.) In a way, the transition is not over yet, and won't be until the end of
Doomsday Clock. So this is their opportunity to include what they consider to be the best aspects of their heroes, their history and backstories, their universe, in a new main timeline (and whatever subsidiary parallel Earths might exists alongside). And they can think it through and plan it out, so there aren't any glaring gaps or conflicts, and give us a DCU with an organic, coherent feel. I don't expect perfection, of course, but I'd love to see a good-faith effort.
But that's just me.