Hollywood blockbusters are all about connected universes now. Marvel set the trend and has maintained the status quo for how to create an interconnected universe spanning multiple movies and now even TV shows. DC has had their own attempts at a shared universe, with mixed results. It's a very hard balance to strike, and it can be difficult to keep up the continuity of a cinematic universe where every story has to connect to at least one of the others. Even the MCU has struggled with this in the past, sometimes leaning too hard into the shared universe side of things and failing to create stories that hold up on their own.
That is the fundamental flaw of the cinematic universe model; every story can no longer exist on its own and be a standalone, it has to fit into the wider story arc of the franchise and all of the characters need to be able to go between stories. These days, it's rare that studios invest in superhero movies that are not connected to any other movies and are their own standalone stories. That is, until The Batman was released and proved that superhero movies can still be massive hits even if they have no connection to a wider cinematic universe.
Batman has shown up on the silver screen in so many iterations that it's getting hard to keep track of them all. DC attempted to slot him into a shared universe with Ben Affleck's version of the Caped Crusader (with Batman v Superman and Justice League), but these efforts fell a bit flat and it never worked quite as well as the Marvel model. The narrative around the current climate of superhero movies is that there needs to be a cinematic universe in order for them to be successful in any way. It's like studios assumed that because the MCU was so beloved, it meant that audiences couldn't bring themselves to care about a story unless it connected to every other superhero in the canon.
However, The Batman was an instant success among both critics and general audiences, and even many die-hard Batman fans agree that it was one of the best portrayals of the character in years. The Batman exists outside of the current DCEU, and for the sake of its own success, it should stay that way. The tone of the movie is so different from the other movies DC has put out (yes, even Christopher Nolan's Batman films feels much different than Matt Reeves's take on the character), and it has distinguished itself in that way. The Batman not fitting into the rest of the DC universe is exactly what makes it unique.
There is a possibility, perhaps even a likelihood, that The Batman will spawn a series of movies and its own Batman universe. And this would be fine, because it would still confine the story to the character of Batman and the Rogues Gallery surrounding him. It would just be difficult, and probably a bad idea, to try and slot this version of Batman into a wider DC universe, like having him in the Justice League. In fact, having The Batman be its own standalone story (or contained series of stories) would actually help set it apart from other current superhero movies, and particularly from the MCU. The tone of The Batman is already incredibly distinct, and the way that it's treated as a franchise movie should follow suit.
There has been no official confirmation of a sequel to The Batman, but at this point, it seems inevitable. This is still a world where the idea of a superhero film that was a super success not getting at least one sequel is unheard of, and Warner Bros would never let an opportunity like that slip through their fingers. Though this story could absolutely be its own separate thing, having a sequel wouldn't ruin it by any means. However, the fact that it even could be a movie that stands on its own is something that is hard to find within superhero franchises these days. It's a testament to the story and to the creative team behind it all that it manages to do what most modern Marvel movies cannot.
While it can be fun to have an interconnected universe where characters from different stories are interacting with each other, it isn't the only way that superhero stories can be told. The Batman proves that superhero movies can stand on their own and still tell a powerful story and be incredibly engaging to watch. Whether or not the world of The Batman gets expanded to include more of the essential features of Batman that fans know and love, it would be wise for DC and Warner Bros to keep this story disconnected from any of the other DCEU projects they've already set up or are setting up for the future. The story and world work well on their own, and trying to make it fit within the confines of a shared universe would only take away what made it so unique in the first place.