The 200-500 is an amazing lens, and a steal for what it offers. Without spending at least four times as much as it costs, there is no better option I know of. (See
this post for some comparative shots and my reason for not owning it.)
Nikon's 1.7x teleconverter may not be a good choice with it, though. As Bikerbrent points out, you could run into serious AF issues, plus it has a reputation as being Nikon's softest TC. The latest incarnation of the TC-14 is pricier but sharper, so IMHO it would be a better choice (but still give you slower AF in combination with some IQ degradation).
I owned a TC-14 for my 300mm f/4 PF for a while and sold it again because I found that scaling up images in Photoshop gave me the same image quality as shooting with the TC. I then bought the TC-20. Returning from my India/Nepal trip and looking at some comparison shots I took, I decided this week to sell it again because the same is true here: no IQ improvement over careful software scaling, so what's the point? This may only be true for my lens and others may have different experiences, but a sharp long lens still looks like the best answer to me.
Wildlife pros go to great lengths to get close to the animals. We others may have to do with lots of cropping.