I am a nature photography, covering te full frame from macro to scenics to animals. If I could pick anything to photograph it would be birds with animals second. Thus I want 2lens- the new 200-400 and 600 M2
I'll jump on board with the 200-400mm f/4 1.4x as the lens I covet the most. It looks to have exquisite IQ and extreme versatility. Will I ever buy it? No. It would be silly for me personally to spend that much for a single lens.
Here's the part where I contradict myself. I can imagine a scenario where I try to discipline myself and over the course of a year: don't buy any other lenses, don't buy any computers, tablets, phones, televisions, etc. I could probably rationalize buying it then. Time will tell...