They can re-edit scenes by either adding or deleting footage already shot, or they can go back and reshoot certain things. Lord of the Rings went back and reshot certain things after principle photography wrapped up. DC is rushing all their films. The Flash and Aquaman haven't even had origin movies, or Batman. Flash and Aquaman have small roles in Batman v Superman, not revealed how big a role. But, thinking they jump in at the last ten minutes of the movie to help Superman, Wonder Woman and Batman take down Doomsday.
The Flash tv show is tackling the multiple Earth premise with Zoom. So, its probable that they might start bringing other characters from Multiple Earths. How cool would it be if they did the Crime Syndicate? And, it seems magic is not being integrated with the shows. Counting down the episodes till they finally bring in Zatanna. If they do, they should bring back Serinda Swan to reprise the role. Would be interesting if they start bringing in some of the Titans like Raven, Cyborg, etc.