DCEU is doomed as a whole!

He did, but we don't know when. That could be on JL Part 2 or 3. Secondly, time travel doesn't make a movie bad.

Knowing the outcome is a minor quibble. A good story, and great writing, can overcome knowing the ending. It is what brings people back to the same stories.
This.
All for a box of chocolates

it's a self-fulfilling storyline. We now know what is going to happen. This is what doomed all the star wars prequel movies in addition to horrible acting.
What doomed the prequels was bad writing. Knowing the outcome is a minor quibble. A good story, and great writing, can overcome knowing the ending. It is what brings people back to the same stories.
In The Godfather II, we know that Vito becomes Godfather of the Corleone Family, but his story is interwoven with Michael's in such a way to be fascinating. The Scorpion King is a prequel to The Mummy Returns, yet it remained a wholly entertaining film. Wizard and Glass by Stephen King, being a Prequel to the rest of The Dark Tower series, is a fantastic love story intermingled in with the rest of the lore the series.
Listen to themthe children of the night. What music they make!

Flash travels back to warn Batman.
Now the whole storyline is focused on getting back to that point of Batman sending flash back into the past to warn him.
it's a self-fulfilling storyline. We now know what is going to happen. This is what doomed all the star wars prequel movies in addition to horrible acting.

But who said time travel will be involved in this film? You're not thinking long term

I have a feeling that's the route they're going, but I hope not. Unless they decide to create another timeline, then things would get interesting

Flash traveled through time to deliver that future memory to Bruce BECAUSE everything went wrong. If things go a better way (batman finding the members because of Flash's warning) them flash won't have to time travel.