In the comic books published by DC, the Monitors are a race of all-powerful aliens who watch over the multiverse itself. They have been involved in the biggest cosmic events, such as Crisis on Infinite Earths and Final Crisis. Originally, the Monitor was created by Marv Wolfman and George Pérez in 1981 to lay the groundwork for Crisis on Infinite Earths. The storyline itself is important because it streamlined DC continuity: by that point, there was an infinite numbers of parallel Earths, and the cosmic event ended with all of these Earths merging together to create a single Earth with a single continuity. Now, characters from other worlds were retconned into being a part of the main world of DC Comics, where they were free to interact with one another.
This has us wondering if this is part of the storyline we will witness in the Arrowverse's "Elseworlds." After all, with the Monitor involved and an emphasis on alternate worlds, it seems quite likely that we might be looking at some sort of Crisis. What's more, such a crisis was already hinted at in the pilot episode of The Flash. Viewers might recall a newspaper article from the future (the year 2024, to be exact) where the Scarlet Speedster had mysteriously disappeared following a crisis that involved red skies. If Crisis on Infinite Earths is known for anything else, other than merging DC continuity, it's because it featured the heroic death of Barry Allen. Now, it's already been established that the timeline on The Flash had been put on the fast track and altered, which could mean that the year 2024 might not be set in stone. Instead, it could very well end up being in 2018.
This doesn't mean that Barry Allen will die, but it he could get lost in the Speed Force (again). Either that or, with history altered, another Flash (or DC character) could end up making the ultimate sacrifice (looking at you, Superman.) It seems like all of the elements are in place for the next Arrowverse crossover to actually be a full on Crisis-level event. If that is the case, then it's likely that, after four years, we will finally see a merging of Earth-38 and Earth-1. If that happens, then Supergirl will finally be a part of the same universe as Green Arrow and the Flash.
That way, the characters will no longer have to resort to crazy universe-hopping shenanigans to hang out. If the characters finally all find themselves on the same Earth, then they will be free to interact as much as they want -- and this can only mean that the crossover possibilities will be easier, and much more frequent. And that can only be a good thing.