Officially, there will be a four way crossover between all the Arrowverse shows, and not only that but our newest addition to the Arrowverse, Supergirl, will be the focus unsurprisingly. Due to her being the newest addition (whether it will be on her separate Earth, or her being integrated post-Flashpoint arc is unrevealed), they want her to be the main focus, maybe so that people who may not watch the show may get familiar with the character. We only know a few things for sure, it will take place over four nights (YAY!) and will not be a musical (thank God). Also Marc Guggy had this to say about what we should be expecting:
“It’s an external threat from outside the shows that will involve Supergirl coming over to The Flash, then Arrow, then Legends of Tomorrow.”
While we have already seen her interact with the Flash, it will be interesting to see how she’ll interact with the incredibly grim Green Arrow, and the time traveling team of the Legends. Since it is also an external threat, it won’t be Prometheus from Arrow, the Legion of Doom from Legends, and whoever the big bad will be in The Flash and Supergirl. The article also stated that while Supergirl will be heading to each show, it is unclear whether or not the other three shows will be interacting with each other, or if it’s just Supergirl heading to each show to help with the big bad. I wouldn’t mind either, and I think it would be interesting for Diggle from Arrow to see a guy with just super speed then see someone like Supergirl, so we just have to stand by for now. I think it would be kind of a cop out just to have Supergirl show jumping without any interaction between the characters from the other shows. I think it would be cool if Supergirl interacted with Flash then Arrow, then have Legends into the mix, then at the end of that episode, be all together as four shows. I think that could be an interesting way of doing it.
Source: Screen Rant