We're told that demo versions of the upcoming replacement for the EF 100-400 f/4.5-5.6L IS have hit select photographers hands for field use.
The new design resembles the EF 70-300 f/4-5.6L IS with a thin focus ring at the base, and a wider zoom ring near the front element. The aperture range for the lens will remain at f/4.5-5.6. Another mentioned feature is in the lens hood for the lens. It sounds like there will be a retractable opening near where a filter would be, so you could turn your circular polarize easily if you needed to.
As with anything regarding this oft rumoured lens, we take things with a grain of salt. That being said, things are lining up with a lot of consistent information.
Hoping to hear more soon.