I think a new 500mmf/4 is missing you seem to have the 300 covered, but have nothing on the long end. I have used the old 500 it is handholdable but better with a monopod which is fine for travelling. The new version is considerably lighter and has better IS. Rather than reaching for a teleconverter with a 300mm have the 500 ready to go.
We would all love a wild card that would let us consider a 400/2.8, life is short play the card with a 400 or 500.