Early pieces of concept art for Aquaman reveal a scrapped battle sequence with Batman. Warner Bros. and DC Films tried to quickly build a comic book cinematic universe, but had some problems along the way with the long-term vision and reception. It was Wonder Woman that first showed how these movies could exist in a larger universe without needing to go overboard with connecting it to other films, and Aquaman took that example and ran with it.
The solo movie for Jason Momoa's version of the underwater hero was almost completely devoid of any references to the larger DC universe. Outside of one quick namedrop of Steppenwolf, Aquaman stands all on its own with the story and characters it uses. Before the film came out though, many wondered if this would actually be the case. There were rumblings that a Black Adam Easter egg or cameo could be included, while many thought Gal Gadot could jump in for a small role after the popularity of her own solo film.
Thanks to concept art from Aquaman shared by @AquamanUniverse, we now know that at one point they were kicking around the idea of Batman appearing during a major set piece. The appearance would've come during an invasion of Sydney, Australia that would make for a giant battle of its own. The third piece of art is titled "Batman Arrives" and shows the Flying Fox coming to Sydney, with Batman controlling the aircraft based on the title.
Since it isn't known how seriously director James Wan and the team behind the film were considering including this sequence, the best way to try would be to look at what Aquaman was when it was finished. Considering how large of an action set piece this would be, it's extremely unlikely that this was ever filmed (especially with Ben Affleck's status as Batman up in the air when Aquaman was being filmed). The movie also never shows Atlantis actually invading the surface and starting such an attack, so this concept art appears to be from very early on in the developmental process that ended up being scrapped as Wan's vision took shape.
While the possibility of Batman making a cameo appearance in Aquaman sounds exciting on paper, it was probably for the best that it didn't happen. Not only would the uncertainty with Affleck make this an odd case, but it also just allowed Aquaman to live on its own. The final product doesn't organically have a place for Batman or other Justice League members to appear, and Wan didn't seem too interested in working in such appearances anyways. But, this could also be somewhat of a missed opportunity to see Batman and Aquaman on the same screen again. With Matt Reeves set to reboot the Caped Crusader with a younger version, it could be a while before they cross paths again, unless future films retcon in an earlier encounter between them before Justice League.
Source: Aquaman Universe
