After two successful crossover events and subtle hints that Gotham City exists in their universe, Arrow is taking the next major step. The next mega event that will crossover all of the CW DC shows will finally show Gotham and introduce Batwoman.
The news was released by Stephen Amell himself at The CW Upfront presentation. Details on what the crossover storyline would be were not released only that Green Arrow and others would travel to Gotham and encounter Kate Kane aka Batwoman.
Considering Superman has now appeared on Supergirl, it is entirely possible this could mark the beginning of other members of the Batfamily appearing on Arrow, perhaps even Batman himself.
