According to dpreview forum user, he just some confirmation from NPS user that Nikon will soon announce a new AF-S NIKKOR 600mm f/5.6E PF ED VR lens. This lens will also be a compact lens similar as 500mm f/5.6E PF ED VR lens (Amazon, B&H Photo Video, Adorama).
From dpreview:
“I met up with an NPS member today to lay hands on his 500PF. He had it on a D5, so it was quite back heavy. But I liked the weight and it’s clear that it would be better in tough conditions than the 500FL. When I told him I intended to stop on my way home and order one, he told me he had it on good authority that Nikon would be releasing the 600PF this summer.”