An update to the 100-400 has been rumored for years. It sort of reminds of the 24-70 replacement rumors. Apparently patent applications for optical formulas don’t guarantee a speedy release. You can see another patent here.
A very solid source says as it stands, a new 100-400 will be announced alongside the EOS 7D Mark II in the late summer/early fall. Even if the availability of the 100-400 is not immediate, Canon feels the two products will fit extremely well together from a marketing standpoint.
Just a hunch about pricing, it won’t be cheap. See Nikon’s new 80-400…… :)