So I am not the only one
Please Canon develop us a new 400 f/5.6 with IS and make money with it !
Would you still love it just as much if it weighs 25% more? And cost twice as much?
The current lens is almost perfect the way it is, like, dead on, they did such a good job 20 years ago that no-one since has bothered to compete.
If you have any need for a 400mm lens, just get one. With the complete lack of competition they probably won't upgrade it. Ever.
As long as we're dreaming though.
I would be interested in a DO upgrade, cut a few inches off that bad boy, and make it sharper, give it real weather sealing, and you have something that the competition won't be able to touch for another quarter century.
Then add IS in 2040.