The Flash, or at least Barry Allen, started out on Arrow, so it's no surprise that the two shows crossed over a number of times last year, with some really excellent results. Needless to say, that's something they need to continue with this year, and now they can go even bigger and better with them. DC's Legends of Tomorrow will be a spin-off of both shows, with characters such as Ray Palmer, Sara Lance, Leonard Snart, and Martin Stein. Before that debuts in 2016, those characters can appear on Arrow and The Flash, and continue to do so afterwards (and vice versa), opening the door for some pretty epic three-way crossovers. In terms of Arrow specifically, this will really help expand the show's universe, which has tended to be more grounded in reality so far, with the inclusion of some time-travel elements and even possible alternate dimensions, as they'll be coming into play on both Legends of Tomorrow and The Flash.