I expect that in response to Vega, we'll see a 1080Ti as a 'full fat' (or close enough) GP102-based card arrive as the range-topper, and the 1080/1070/etc move 'down' a price point (if necessary) to match Vega. I suspect Vega will launch pretty close in price/perf to Pascal anyway: as seems to be the pattern, sitting a bit behind in everything except Vulkan/DX12, and trading blows for DX12/Vulkan depending on how any given game has optimised their engine.
Volta is probably quite a way off, and IIRC has already been slated to turn up in the HPC role quite some time before consumers get it. Summit will be gobbling up ~27,000 dies at full rollout, with Sierra grabbing another ~8,000. That's a lot of production volume dedicated to dies on a new architecture (and big ones too). Depending on whether Nvidia can find a HPC market for the binned dies from that production run, the 'off cuts' from that production push may end up in other supercomputers, or possibly make their way as a 'halo' card (I'd put this as more likely if yields are very good and there are few rejects). A year of HBM2 volume production for HPC may push prices down enough that it can make its way to consumer high/upper-mid range cards, but no game as of yet has been anywhere close to memory bandwidth limited, so it may not be worth bothering with anything other than GDDR5X or GDDR6.
On the normal consumer side, I could see a 'Pascal 2.0' refresh slot in before Vega for the consumer side of things unless Vega really pushes performance forward. Probably no change in process, so more in the way of optimisation (Similar to Maxwell => Pascal) and added features (likely more VR stuff. native non-rectilinear processing would be awesome, but unlikely to be possible with just a minor update).
