It was announced this morning that Grant Gustin (The Flash) would be making a guest appearance on Supergirl on March 28. This is something that had been rumored in the past, but due to The Flash being on CW and Supergirl being CBS no one thought it would happen. Greg Berlanti and Andrew Kresiberg commented in a joint statement
We are so incredibly excited to announce something that we have dreamed of happening since we starting making Supergirl. We want to thank Grant Gustin for making the time to come visit (on top of his already immense workload) and all of the folks at CBS, The CW, Warner Bros. and DC for working this out. And finally thanks to the fans and journalists who have kept asking for this to happen. It is our pleasure and hope to create an episode worthy of everyone’s enthusiasm and support.
As of right now there are no story line details on the cross over but this is a huge breakthrough and if this works then we may see more crossovers in the future. If Flash and Supergirl can exist in the same universe, then there are also multiple Earths, Arrow and his team, also the existence of the team from Legends of Tomorrow and their time ship. The annual Flash/Arrow crossover could turn into an Arrow/Flash/Supergirl crossover.