Despite the push/pull design... what's so bad about the current 100-400ÃŸ?
Really poor handling in multiple ways, poor performing IS, optical quality degrades a lot if the IS happens to be at a substantially off-center position when you shoot, mediocre corner performance, relatively slow AF compared to newer lenses, not diffraction-limited in the center wide open despite being f/5.6.
I'd really, really, really like to see a new high-pixel-density 1.6-crop camera with high-performing f/8 AF sensors, and a new 100-400L that goes with it that's designed to be dead-sharp wide open with a 1.4x TC on it on that camera (which could AF with it that way), and to have all that other stuff fixed. The ideal one would be f/4.5 at 300mm and have AF faster than the 400/5.6L while weighing the same, thus eliminating the need for the 300/4L IS and the 400/5.6L.
That system would be virtually the ultimate hand-holdable "reach" camera system for good light applications.