It seems that there is another Arrowverse crossover coming and I'm not talking about the Supergirl / Flash musical episode. According to executive producer Marc Guggenheim, next week's Arrow – Second Changes, will feature the fastest crossover with the flash ever. They way he phrases it sounds like Barry Allen may race into Star City and drop off… maybe the new candidate for Black Canary as that is one of the two main threads for the episode. The other is the flashback where we find out more about Talia al Ghul. Maybe all we'll see is a red blur.
— Marc Guggenheim (@mguggenheim) January 27, 2017