A highlight of both of The Flash and Arrow was when the two shows would crossover with each other, starting with brief appearances in the first episodes of the season, before a proper two-part crossover Flash v Arrow/The Brave and the Bold. Oliver Queen appeared again in the penultimate episode of The Flashs season, while Barry helped out in the season finale of Arrow, and supporting characters had crossover moments too, with the likes of Felicity and Cisco appearing on the other's show. Theres already an established chemistry between them, and seeing the characters on the others show provides a chance to see them in a different light. Thats something that will no doubt continue in Season 2 of The Flash, and season 4 of Arrow, especially with new spin-off on the block Legends of Tomorrow. It features characters from both Arrow and The Flash, such as Atom and Captain Cold, so it opens the door for crossovers between all three shows. Theres also the slight possibility of a crossover with Supergirl, which is set to debut around the same time. It shares a lot of the same back-room team as Arrow and Flash, so could make an appearance if the networks (Supergirl is on CBS) can come to some sort of agreement.