The Flash ending was one of James Gunn's first changes when he became DC Studios' Co-CEO along with Peter Safran in October 2022.
When Gunn was put in charge of DC Studios, he had to make important and controversial decisions. DC's connected universe had to be more cohesive to compete with Marvel Studios, so Gunn needed to reset it with a new slate of films. This included replacing some of the actors who were there since the start of the DC Extended Universe. When the news of these decisions was announced, they were met with praise and heavy criticism.
DC's latest film, The Flash, was already in post-production when Gunn and Safran became Co-CEOs. The film's ending needed to change to align with their vision for the DCU's future. The film's original ending was similar to the final version but showed Supergirl, played by Sasha Calle, and Batman, played by Michael Keaton, on the courthouse steps. This was screen-tested multiple times but was changed when WB and Discovery merged to add Henry Cavil's Superman and Gal Gadot's Wonder Woman to the scene.
This ending conflicted with Gunn's plans for the DCU, as he was unsure if either Gadot or Cavil would return. The Flash ending had to be changed one final time. This was the ending that audiences saw in The Flash. Instead of the other characters mentioned before, fans see Barry Allen greeted by Bruce Wayne outside of the courthouse. But this Bruce Wayne is played by George Clooney.
Clooney played Batman in Batman and Robin, which is widely considered to be the worst Batman film. Even Clooney has said that the film was a complete mess. However, this ending does not clarify if this is the same universe as Batman and Robin. This could be another universe where Clooney is Bruce Wayne. The Flash's ending does enough to tell the audience that the universe has changed but also gives Gunn and Safran room to decide what's next for the DCU.
The Flash is now playing in theaters.
Source: The Hollywood Reporter