It has rumored for a long time that Canon will announce two new big white lenses ahead of Photokina 2018 in September. One of the big white lenses is now confirmed, it is the Canon EF 400mm f/2.8L IS III USM Lens.
The other super telephoto prime lens is unknow yet, but it is also a verison 3 lens, which means it could be EF 300mm f/2.8L IS III USM Lens or EF 500mm f/4L IS III USM Lens or EF 600mm f/4L IS III USM Lens.
The main upgrades for these lenses will likely focus on weight reduction and updated coatings. The optical formulas is rumored to remain about the same.