I would trade my 70-200 f/4L IS for a 100-300 f/4L IS
Based on other posts, this may put me in a minority, but I'd seriously consider selling my 70-200 2.8 for a 100-300 4.0. My 70-200 spends a lot of time with the 1.4x on it anyway, and a 100-300 would complement my 24-105 4.0 very well. Performance of the 2.8 zoom with the 1.4x is very good, but I'd love to take it out of the optical path. My rare need for f/2.8 could be met by an occasional rental.
On the supposed 300mm 2.8 replacement: Anybody besides me hoping they do away with the removable tripod collar? That's just a very expensive accident waiting to happen in my opinion. The copy of the current IS-version 300mm 2.8 that I rent locally from time to time has wear on the interior surface of the collar that makes rotation stiff and gritty. That never happened with the original non-IS version of this lens that I forever regret selling.
If anybody ever removes the collar from this lens, can you tell me WHY? Even in the rare instance when I'm not using a monopod with that lens, the tripod mount rests perfectly in my palm and holds the lens barrel just far enough away from my hand for easy fingertip manual focus.