If Canon were to release a new 400 f/5.6L with all the latest features (v. 4 IS, weathersealing, modern build quality and materials, better MFD, etc) what do you think the cost would be? I think it would have to remain below the price of a 70-200 IS II, but I could definitely see it coming close to $1800 or so given Canon's recent price structure.
Considering the 400L f/5.6 USM is around US$ 1300 and the 500L f/4 IS II USM is US$ 10,400ish and the 400L f/2.8 USM IS II is around US$ 11,000 then I would imagine that any new lens in the 400 f/5,6L range would be somewhere in the middle.
Don't forget that rather silly 100-400 with the built in extender is rumoured to cost a staggering US$ 10,XXX. Which imho is just mad.
But like someone said, if Canon were to do such a lens it is more likely to be f/4 than f/5.6 as most would want to stick a TC on it.