During an appearance on SiriusXM's The Jess Cagle Show with Julia Cunningham, Keaton explained how his return as Batman came about, and expressed his excitement for his role in The Flash:
"[Batman] was what it was. [...] And then [Batman Forever], I just couldn't do. I just thought, 'Ooh, this is not good. Not good. I just can't do it.' [...] So, I walked away. And then, as time went on, it's like I caught up with the culture, I guess [...] Everything that's happened in that world — the superhero world, and [...] the zeitgeist of all that stuff, I was really out of it, to be honest. And at some point, you're a jerk if you don't go, 'Well, this is a legitimate world. You're not better or worse than this.' [...] So, when it came around again, first of all, I was getting curious about it. I thought, 'Boy, what would that be like?' [...] And then, coincidentally, there [were] murmurs. I got a call from Warner Bros. They wanted to talk to me about something, and there [were] hints of Batman, but I think it was about some other thing regarding Batman. I don't know what it was. [...] And I got a call later saying, 'Hey, [we want you] to read something.' And I just thought, 'Of course you have to do this, but it has to be good. There's no reason to do it if it's not good.' [...] So, it's really good, and I just jumped in and had fun. And why would you not? Director Andy Muschietti is fantastic and really creative. I don't know... it's fun."
Given that the actor shared a picture of himself on the Batgirl set, it's safe to say he is indeed having fun reprising Bruce Wayne for the big screen.
Batgirl does not have a release date yet, but it's expected to come out on HBO Max in 2022.