When shooting wildlife in open plains even the 600 falls short many times. So the 200-400 and 800 seem ideal to me.
Maybe if Canon releases an 800/5.6L IS II. But I'll take the 600 II over the current 800. With the 1.4xIII on the 600 II, the 840mm f/5.6 combo delivers better IQ than the current 800/5.6, plus the new 600+1.4x is lighter, and allows me to use all 61 AF points on my 1D X (the current 800/5.6 is a Group F lens, allowing only 47 of the 61 AF points to be used). The 600 II + 2xIII is also optically better than the 800 + 1.4xIII. Given that, I expect we'll see a new 800 II coming along pretty soon, because the 600 II seems to have made the current 800/5.6L IS obsolete.