Some more information on next-gen graphics cards has arrived thanks to a press release from SK Hynix about their new HBM2 tech, with SK Hynix also shipping GDDR6 memory in Q1 2018.
SK Hynix was also at NVIDIA's recent GPU Technology Conference showing off their 8Gb GDDR6 module with 16Gbps of bandwidth, up from the 11Gbps of bandwidth available on the fastest GDDR5X on GTX 1080 Ti. GDDR6 is listed as a Q4 2017 product, meaning it will enter mass production in Q4 2017 for a huge Volta launch in Q1 2018 - at least according to WCCFTech.
I think we'll see a Pascal refresh based on GDDR6 at first, and then a full Volta line up with a blend of GDDR6 and HBM2 technologies - GDDR6 for the GTX 2080/2070, and HBM2 for the Volta-based TITAN Xv, and what I'm hoping for: a dual-GPU Volta graphics card.
NVIDIA will most likely launch a GeForce GTX 2080 (or GTX 1180) with 8GB+ of GDDR6 @ 14Gbps on a 256-bit memory bus, getting close to GTX 1080 Ti performance. The full monster Volta card will probably launch with a 384-bit memory bus and 12-16GB of GDDR6 @ 16Gbps, while a TITAN Xv could unleash a full 512-bit memory bus, or shift over to HBM2 depending on the cost/scarcity of HBM2 from Samsung.