Sony recently has filed two super telephoto prime lens patents along with Sony FE 400mm f/2.8 GM OSS Lens (B&H Photo Video). This two lenses are Sony FE 500mm f/4 GM lens and Sony FE 600mm f/4 GM lens. The announcement of these two lenses could be in 2019 to 2020.
Sony FE 500mm f/4 GM lens patent:
Sony FE 600mm f/4 GM lens patent: