Per Entertainment Weekly, Arrow's Stephen Amell made the announcement on The CW's Upfront presentation today that Batwoman will be featured in the next crossover event of the DC superhero series. Mark Pedowitz also added that they "are adding the city of Gotham into the Arrowverse", so theres also that.
Arrow star Stephen Amell made the announcement at The CW’s upfront presentation on Thursday. “We’re incredibly excited to announce that we’ll be doing another crossover event this fall on the CW, and we’ll be introducing a new character,” Amell said. For the very first time appearing, we’ll be fighting alongside Batwoman, which is terrific. The crossover is going to make it to air in December. I need to leave right now and start filming it.”
@StephenAmell is here to promote the arrival of Batwoman and the city of Gotham to the Arrowverse. @ArrowCWTV @TheCW #cwupfront #upfronts @MediaVillageCom pic.twitter.com/0DxvP4gGMj
— Ed Martin (@PlanetEd) May 17, 2018