It's been three years since The Batman premiered and the idea was that shooting for the sequel would start around now, with a premiere in the autumn of 2026. But... that didn't happen.
The script dragged on and possibly also interfered with James Gunn and Peter Safran's plans for the DCU. This led to a major delay and instead the sequel will not be released until October 1, 2027.
During a film event in London, Batman himself, Robert Pattison, commented on The Batman - Part II and told Deadline that filming will start towards the end of 2025. He wouldn't say anything about the story, but added that "it's cool, though, it's cool". There is, however, a strong possibility that the Penguin, played by Colin Farrell, will appear in part two, although he is not believed to be the main villain.