I have a theory. I believe that the DC Extended Universe will cross over with the DC television series. I know what you're thinking. "Warner Bros. already confirmed that they won't! They already have Ezra Miller cast as Flash instead of Grant Gustin!" But trust me, it can still happen and here's why.
DC confirmed that the movies and tv shows exist in different universes. However, did they say they are not within the same MULTIVERSE?! They have not said this! And what's a big thing that's been featured in the DC Universe for so long?! THE MULTIVERSE! Some of DC's biggest storylines ever have included the multiverse, notably the Crisis on Infinite Earths.
Okay, so they are in the same multiverse, but how's that going to get them together? The answer to that question is the Flash. One reason I think this will happen is because DC has been, for the most part, pretty hard on CW for using characters that they want in movies. In Arrow they had to change original plans to introduce Harley Quinn because DC wanted her in the Suicide Squad. They replaced her with Cupid. That's also why they had to kill off Deadshot. That's also why Bruce Wayne has never appeared in Starling City, even though the creators really wanted him to appear.
However, if that's the case how can the Flash get his own TV show if he's going to be in the movies?
The Flash has been known to travel through universes. Most notable done in the Flashpoint comic series. This is why I believe that in both the CW television show, and within the movies, the Flash(es) will cross paths and break the multiverse. This may not happen now, it may not happen for a while. But I guarantee this will happen.