Matt Reeves' The Batman is looking like it will very much be its own thing and stay out of the current DCEU timeline. Additionally, it looks like Ben Affleck will be exiting the role after either Flashpoint or Suicide Squad 2. There have been a lot of rumors swirling around The Batman movie over the last handful of months, but this report seems to be some of the more believable reports that have started to pop up. Matt Reeves revealed at the end of December that he had not even started the script yet, but added that he has the story and an outline ready to go.
Batman on Film reports that The Batman will exist outside of the current timeline of the DCEU. The report goes as far as to say that the upcoming movie may even be announced under the new, long-rumored Warner Bros. banner. This is all speculation at this point, but it makes an awful lot of sense. It has not been confirmed, but it truly looks like Ben Affleck will not have anything to do with the project, especially since reports of new castings have been popping up. If Affleck is not in the movie, there is no reason to keep it in the already established timeline of the DCEU.
Another reason that this report makes sense is because Warner Bros. and DC just made some huge behind-the-scenes changes that are rumored to stream-line their processes to stay out of the way of the filmmakers while making the upcoming movies that are currently in development. This "less cooks in the kitchen" approach should help to make movies that are currently in production less cluttered and, ultimately avoid what happened with Justice League and Batman v Superman. Keeping The Batman separated from the already established DCEU is a great idea for a fresh start.
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The report also states that Ben Affleck will be exiting the role soon, which has been talked about for nearly a year now when he stepped down as director and later when Matt Reeves announced that he was starting the project over from scratch. The report suggests that Flashpoint, which has also been reported about, or Suicide Squad 2 will be the last time that we see Affleck as the Caped Crusader. It's a toss-up here, but it looks like Affleck could stay on board for both movies, which still hinges on if Flashpoint ever makes it past the development stage.
With the new reports of changes behind-the-scenes at Warner Bros. and DC, coupled with what director Matt Reeves has said about starting from scratch, it could very well mean that The Batman will exist outside of the current DCEU timeline. This is all speculation at this time, so we'll take the report with a grain of salt until any more news comes out. You can read the original report about The Batman taking place outside of the DCEU courtesy of Batman on Film.