Sony FE 300mm f/2.8 GM OSS lens for Sony E-mount is already listed at B&H Photo. The lens development was announced back in January and the official release should be coming soon.
"A new FE 300mm f/2.8 GM OSS Lens from Sony is under development and will feature the incredible high-resolution and spectacularly beautiful bokeh true to Sony's acclaimed line of flagship G Master lenses. Made for the Sony E-mount, this lens will deliver an elevated shooting experience for professional sports, news and event photographers, and videographers looking to capture decisive moments with higher image quality. The lens will expand on the design of the 600mm f/4 and the 400mm f/2.8 Grand Master OSS lenses."
Sony announced the development of a new 300mm f/2.8 G Master lens
Via SonyAddict