If there's one thing we can count on in the DCCW universe, it's that the various shows will crossover with each other, every year, like clockwork. For example, we already know that Felicity's going to show up in The Flash this year, and Green Arrow will visit the Legends of Tomorrow. But now we know a bit more about the season's biggest crossover. Executive producer Marc Guggenheim offered up this information about all four DC shows:
It's an external threat from outside the shows that will involve Supergirl coming over to Flash, then Arrow then Legends of Tomorrow. I don't like to oversell. She is going to be coming over but we're not going to do a full on Supergirl crossover episode. I don't want to give you the wrong impression.
Which makes sense, since Supergirl is the show that airs earliest in the week (on Mondays). It sounds like whatever new threat there is will show up in Supergirl, and she'll visit the other shows to tell them about it. It doesn't sound like she's going to, for example, stick around to fight the threat in any of the shows.
I wonder if her appearance will be larger in Flash, which she already has a connection to. Also, Guggenheim's answer doesn't mention if Flash, Arrow and Legends of Tomorrow will crossover with each other -- like Supergirl plays a role on Flash, someone from Flash shows up in the Arrow episode, and so on and so forth.
Regardless, I think we can all agree that the Arrow crew's reaction to Supergirl might be the highlight of that week.