From
The Hollywood Reporter on this season's crossover event:
Ever since it was announced earlier this summer that this year's annual CW crossover event would introduce Kate Kane aka Batwoman (Ruby Rose), speculation ran wild as to how the Green Arrow (Stephen Amell), The Flash (Grant Gustin) and Supergirl (Melissa Benoist) would end up meeting the Gotham City hero. With the latest bit of casting news announced earlier this week, some light has finally been shed on the catalyst for this year's Arrow-verse crossover. LOST alum Jeremy Davies has been cast as Dr. John Deegan in the crossover, who interestingly enough has no DC Comics counterpart. Davies will have free reign in creating an all-new villain for the huge crossover event. According to a press release, Dr. John Deegan "is a doctor at Arkham Asylum For the Criminally Insane in Gotham City, but he might just be crazier than the inmates he treats. His machinations will draw Green Arrow, The Flash and Supergirl to Gotham City." So that's how all the heroes end up in Gotham City, aka Batwoman's domain, as Deegan's actions will be so far-reaching that one, possibly three, superheroes will have to travel to a different universe.
Because it's still unclear where exactly Gotham City is. This latest announcement still doesn't explain which universe the Arrow-verse heroes will travel to. In the past, Oliver Queen has name dropped Bruce Wayne and Gotham on Arrow, but he was only referring to Earth-1's versions. With Supergirl's involvement in this crossover, there will be some universe-hopping already to get her in the same room as Oliver and Barry. But is she the only one traveling to a new universe, or will Kate Kane's version of Gotham City be in a new universe, meaning that Oliver and Barry need to breach to a new universe along with Kara? If the potential Batwoman series is greenlit after the crossover, having it set in the same universe as Arrow as The Flash means more opportunities for smaller crossovers in the future. If it is set in a different universe like Supergirl, the reasoning for crossovers in the future need to be bigger, meaning less opportunities for quick one-offs.