1) Anti-Monitor
Ever since The Flash season 1 teased that the Scarlet Speedster would disappear in a “crisis” at some point in the future, fans have been calling for the Arrowverse to do its own version of the iconic storyline Crisis on Infinite Earths. And, to do the story properly, they would need to bring the Anti-Monitor into the fold.
It’s a lot more complicated than this but, basically, the Anti-Monitor is a phenomenally powerful cosmic being from the anti-matter universe. He attempted to go back to the point of creation and remake the entire multiverse in his image. Thankfully, the combined forces of the Justice League defeated him – an event that led to one new, singular universe.
We’re not saying that Crisis should be adapted next season, but just at some point in the future – whenever they feel that continuity has got too convoluted and needs a hit of the good ol’ reset button. Seeing as Supergirl is sitting on its own separate Earth, the Arrowverse is crying out for a Crisis-type event to get everyone on board the same world.