It started with a crossover of sorts. Barry Allen (Grant Gustin) debuted on an episode of Arrow before The Flash was ever given the greenlight. In the shows first season and Arrows third, Barry would go to Star City and Oliver (Stephen Amell) made a trip to Central City. The following year, the first official crossover introduced Hawkman and Vandal Savage and set up the Legends of Tomorrow. And of course last years Arrowverse Crossover brought together all the shows against an alien invasion. Once you go alien, where do you go next? Turns out the answer isn't bigger, but deeper as in emotionally deeper. Executive producer Marc Guggenheim told EW.com:
If we end up doing what we're talking about doing, it's going to be pretty cool. The crossover is always the most challenging thing we do, but it's also the most fun. Every year we feel the pressure [to top ourselves]. I feel like each crossover has topped the previous one. Topping last year's, that's a high bar, if for no other reason than what do you do that's bigger than aliens? We've got some very, very exciting ideas…. Very early on in the process of doing last year's crossover, Greg [Berlanti] said there's probably no way to get bigger than aliens, so the best way to make the [next] crossover especially epic is if you can't increase the bombast, increase the emotional stakes and the emotional payoffs, so that's a very oblique comment on where our heads are at for this season.
Well, anytime someone starts talking about stakes and payoffs you have to start thinking that someone may day. Okay, maybe that's just how I read it. The crossover will be a bit easier this year as they know that Supergirl will be a part early enough to plan it in the schedule and make the full episode part of it. We're told Black Lightning will not be part of the event… at least so far.