Although it may seem surprising today, the producers of the Arrowverse did not set out to create a multiverse of heroes. The Arrowverse encompasses The CW's lineup of DC programs, from Arrow to Legends of Tomorrow. Although not originally part of the Arrowverse, Black Lightning is joining thanks to the upcoming Crisis on Infinite Earths.
One can spend hours discussing the Arrowverse, and how it came from small beginnings. Back in 2012, The CW premiered Arrow, an adaption of the character from DC Comics. Smallville had concluded its run a year prior, so Arrow was the network's new superhero program. The Flash followed two years later, and even eclipsed Arrow in ratings. From there, the Arrowverse would skyrocket with additions such as Legends of Tomorrow, Supergirl, and most recently, Batwoman. Despite Arrow coming to a close this season, the Arrowverse has not run out of steam. Crisis on Infinite Earths is going to be massive. Post-Crisis, fans are highly anticipating Superman & Lois. It's impressive how far the Arrowverse has come. That's why it may seem surprising a multiverse wasn't originally planned.
In an expansive interview with Entertainment Weekly, producers on the Arrowverse discussed the shows, crossovers, and how everything regarding the shared universe came to fruition. According to Greg Berlanti, the Arrowverse wasn't planned, and "each step was a surprise." Marc Guggenheim notes when Arrow premiered in 2012, they were asked if other super-powered heroes would be introduced. The answer, according to Guggenheim, was "No." As fans would eventually see, that changed when The Flash launched. Guggenheim describes the Arrowverse's existence as "Unbelievable."
"The truth is [the unexpected expansion] really speaks to my whole philosophy about building a universe. The best way to do that is to do one good show. That one is really hard. Then if you succeed, do a second really good show."
The Flash season 2 introduced the idea of a multiverse, which means there are alternate universes out there. In Supergirl season 1, Flash heads on over for a crossover. Sarah Schechter notes this multiverse crossover was not "calculated," but rather from a "pure, creative desire" to see Melissa Benoist and Grant Gustin together. The interview would also discuss the various big crossovers, and according to Guggenheim, Crisis on Earth-X is the "gold standard" for the Arrowverse's annual crossovers.
At this point, shared universes in pop culture are nothing new, but it's usually impressive nonetheless how they form. Arrow began as a contained crime thriller, but later introduced viewers to Barry Allen, who then introduced the multiverse. Now Crisis on Infinite Earths, probably the grandest comic book event of all time, is being adapted for TV. The Arrowverse has come a long way since 2012, and doesn't appear to be going away anytime soon.
Source: Entertainment Weekly