After a renewal of the 300mm and 400mm offering of Canon in the L series of pro lenses, nobody will be surprised to see Canon announce that there will be new versions of the 500mm f/4 and 600mm f/4 lenses. Obviously reserved to the pros with deep pockets (for wildlife or sports photography), these lenses will bring the latest stabilization technology to the biggest lenses from Canon.
Availability is still unclear seeing that the 300mm f/2.8 and the 400mm f/2.8 are late on the market and there is no promised availability date for these two new big white glass chunks.
Personal note: I start to wonder whether upgrading to a better Image Stabilization (IS) is worth it on lenses that cannot reasonably be handled without a tripod or some kind of fixed support… Image quality may be a better criteria to judge them.