During the Legendary Pictures Comic-Con panel, CEO Thomas Tull showed a quick video message from Gareth Edwards. In the video, Edwards gave a brief address to the SDCC crowd saying that he was taking a break from Godzilla to work on a "small sci-fi movie" (aka Star Wars), but he wanted to give a quick look at his next monster film. The clip showed grainy newsreel footage (similar to the opening credit scenes from Godzilla) and revealed that Rodan, Mothra and Ghidorah will all appear in future Godzilla sequels. Those three classic Toho monsters will likely appear in Godzilla 2 and future team-up movies and that's an exciting prospect. Nothing else was revealed about Godzilla 2, but I would expect it to hit theaters in summer 2018 thanks to Edwards' commitment to Star Wars. No word on the cast yet since the film is so far out.
Source: The Hollywood Reporter