At the TCAs, it was announced that these will be on the Flash and Supergirl, as a number of the cast can sing. There will be one original song per episode, with the musicals otherwise consisting of pre-existing songs. (This is not linked to the four-way crossover in November.)
Sharon Leal will be appearing as Miss Martian on Supergirl, while Christopher Wood will return to the show as Mon-El, described as a superhero with similar powers to Supergirl from the planet of Daxam. He was an explorer who had landed on Krypton, where Jor-El warned him of the planet’s imminent destruction and provided him with a map to Earth. He suffered amnesia upon landing on Earth. The character originally appeared opposite Superboy in the comics. As far as Cat Grant goes, no decisions have yet been taken regarding how many episodes she will appear in, or whether there will be a replacement for her during her absence.
The showrunners also confirmed that Dolph Lundgren will be appearing on Arrow in the Russia flashback, while Millennium star Lance Henriksen will be turning up on Legends of Tomorrow.
Meanwhile, on CW Seed, Berlanti and Marc Guggenheim are introducing a gay superhero on Freedom Fighters: The Ray. The idea is that whoever is cast as the voice actor for the animated show will appear on the live-action series as well.