We were told a while back that two big white lenses would be announced ahead of Photokina in September. We were also told that the lenses would both be version 3's, which means they'll be replacing two current Big White Lenses.
A different source than the original is telling us that one of the lenses to be announced will be an EF 400mm f/2.8L IS III.
The second big white to be replaced is still unknown at this time
Upgrades for these lenses will likely focus on weight reduction and updated coatings. We expect the optical formulas to remain about the same.
When Canon updated the “Big White Lenses” to version II, they announced the EF 300mm f/2.8L IS II and EF 400mm f/2.8L IS II at the same time and a few months later announced the EF 500mm f/4L IS II and EF 600mm f/4L IS II. We wouldn't be shocked to see things happen the same way for the version 3 lenses.
Update: We've upgraded the rumor rating on the EF 400mm f/2.8L IS III to [CR3]