Act444 said:
Interesting discussion. I've also been seeking more reach but keep putting off the decision...
The 100-400 performs better than I expected, and holds up well even on my 5DSR(!) which is nice. However, 400 isn't enough reach for many birds (on FF) and I find myself wanting a bit more. I thought about both the 400 DO and the 500 f4 II but for the latter, price and particularly weight was a big concern. If the 400 takes TCs well that could be a possible route, but reports of a 200-600 zoom and a new 600mm lens had me pausing (600 would be ideal, looking for something handholdable as I don't want to be bound to tripod).
If price is no object then there might be more to consider. If weight is no object there might be more to consider. If IQ is no object there might be more to consider. If AF speed is no object there might be more to consider. ETC.
Basically, you consider the constraints that you put upon yourself and go from there. For me the 400 DO II was a no brainer once the F8 focus improvements came along. 300 X2 got me shots with the 6D but it was pitiful for AF speed. However the IQ was quite good so it served me for roughly 3 years and taught me that 600 for small birds was good but on the edge - too much cropping that made the IQ marginal.
I reasoned that 800 would bump me over that edge and it has, nicely while being hand-holdable and from Birds as Art etc. I learned that 400 X2 AF with the 1DX2 was excellent and exceeded the 5D4 so that coupled with fps tipped me towards the 1DX2. Of course I had to compromise on the 30 MP but some 1DX2 features pushed me in its direction and I'm satisfied with the compromise. When birds are active that little burst of 3-5 shots at 14 fps is addictive and productive.
1DX2 + 400 DO II + 2X III is all the weight my aging body can handle hiking.

And that's assuming I stay fit
I doubt that a 600 DO is really going to be that light if you carefully analyze the comparative weight savings relative to existing lenses. The 400 benefited comparatively because F4 became an option over F2.8.
A 600 F5.6 would gain similarly but it won't be produced. A zoom would be nice but it will never be light. Built in converters are nice but they bring size and weight too, which you might not appreciate if shooting native FL. Compromises galore!
Jack