The new Sony FE 400mm f/2.8 GM OSS lens is expected to be released in the Summer of 2018. B&H currently lists the lens shipping date as September 2018.
From the press release:
The new lens will feature the incredible high-resolution and spectacularly beautiful bokeh of Sony’s acclaimed line of flagship G Master™ series lenses, while also offering a far-reaching 400mm focal length and F2.8 large aperture. This combination will deliver a new elevated shooting experience for all professional sports, news and wildlife photographers looking to capture all of the decisive moments with higher image quality.
Via SonyAddict