Today I combine the 70-200 f2.8L IS II and the 200-400 f4L 1.4x, or, when I need more reach, the 600mm f4L IS II with the 1.4x/2xIII extenders. The new 100-400 will be of interest though, if it delivers the IQ we are hoping for, meaning (sufficiently) better than the 70-200 with extenders. On size and weight restricted travel, a 5DIII type body, with a 24-70 f2.8L and a 100-400 f4-5.6L, combined with a 1.4xIII extender, is a pretty small, light and versatile package.
Having seen Canon´s latest price entry point for new lenses, I doubt we will see the 100-400 II for less than $2500 though. If it is good enough I would not even be surprised if it started close to $3000. Let´s hope Sigma´s go at the higher end market scares Canon enough ...