Supergirl Moves To Earth-1
The most famous outcome of Crisis in the comics is that it reset DC continuity and allowed for the multiverse that had become so cluttered over the years to be streamlined and made more accessible and easy to follow again. For instance, heroes such as Jay Garrick who had previously lived on other Earths had their reality changed so that they existed on the main Earth.
The Arrowverse has a very similar situation. Initially, Supergirl had to be set on a separate Earth as it aired on CBS. When it moved over to The CW, though, and Kara became a regular face in the crossovers, it began to feel like an unnecessary obstacle that the Girl of Steel hailed from a different world and her friends had to cross dimensions if they wanted to speak with her.
“Crisis” gives the Arrowverse the perfect opportunity to fold Supergirl’s Earth-38 into Earth-1, meaning that Kara, the DEO, the Martian Manhunter and Superman (if he’s still around) will now exist side by side with Green Arrow, the Flash and the rest. It would be such a natural move to make for the crossover that we’re pretty sure this is what’s in the planning.