The MCU introduced the Multiverse in Loki, and is set to embrace the concept of alternate realities in a big way with the upcoming Doctor Strange in the Multiverse of Madness. The DCEU (or whatever you prefer to call it) will follow suit with The Flash, but Warner Bros. seems to be using their take on the Multiverse as an opportunity to retcon certain elements of their shared world.
Not every aspect of the original incarnation of the DC Films Universe will be altered or replaced, however, and a new rumor claims that two major characters from the "Snyderverse era" will appear.
Possible spoilers follow.
According to Screen Geek, both Jason Momoa and Gal Gadot will show up as Aquaman and Wonder Woman in the Andy Muschietti-directed movie. The site doesn't provide any context for their roles, but based on what we've heard, they will only be cameo appearances. These could be the versions of the characters we know, but since Barry Allen (Ezra Miller) will traverse time and different realities, there's every chance he'll encounter his Justice League teammates on an alternate "Earth."
Previous reports have indicated that Barry will form a new Justice League by the end of the film, and, if this is accurate, it sounds like Diana and Arthur could be along for the ride.
Ben Affleck's take on Batman will also feature, but is expected to bid farewell to the iconic DC Comics hero with one final appearance. How will the movie write his Dark Knight out of the DCEU while paving the way for Michael Keaton's version? Well...
All of this should be treated as rumor for the time being, of course, but it's worth noting that major leaks for The Flash have been finding their way online over the past few weeks.