For quite some time there had been rumors that "The Walking Dead" actor Jeffrey Dean Morgan would appear as Batman in the upcoming "Flash" solo movie. However, as the Hollywood Reporter and Deadline now agree, Michael Keaton is supposed to appear as Batman in "The Flash" instead. Although there is no official commitment, Warner and Keaton are in an early stage of negotiations. Allegedly, it has been leaked that Keaton will not play any Batman, but exactly the same one he played in Tim Burton's earlier films. All this is supposed to be possible, because the DCEU will be changed to a multiverse from now on.
Keaton's appearance is far from being a one-off, as the current negotiation paper apparently also provides for future roles for the actor. Similar to Marvel's Nick Fury, Keaton could therefore take on a kind of mentoring role or act as a string-puller in the background.
Of course, these are all exciting ideas, but of course they haven't been verified yet. So, as so often in the film business, the first thing to do is to wait until an official announcement is made by Warner Bros. The US release of "The Flash" is so far 03.06.2022.