This one is pretty much a given, thanks to the popularity of both shows, their shared fanbases, and the success of the crossover episodes last year. They provided some great moments for both series, with lots of action, the excitement of having the two heroes team up, and the fun to be had with seeing the characters slightly out of place with the tone of the other's show. Now, with DC's Legends of Tomorrow in the mix as well, there's scope for even more crossing over. The three shows will continue to be linked, and characters from Legends will be appearing again on both shows before that one starts, and you'd imagine Flash and Arrow will turn up there as well, so we should be in line for some fairly epic crossovers that really play around with the characters and the mythos of all the shows.