We spotted a brand new Sony patent application filed in 2017 and published now in May. It shows the design and specs of three fast super tele E-mount lenses:
The first lens is the already known 400mm f/2.8.
The second lens is the 500mm f/4.0.
The third lens is the 600mm f/4.0.
I think there is a 90% chance Sony will really launch all the three lenses on market. After the Sony A9 launch Sony stated multiple times that they want to get seriously into the sport-photography market. The 500mm and 600mm are the logical next step after the launch of the new 400mm f/2.8 FE lens.
500mm lens design:
600mm lens design: