At the Hot Chips symposium, SK Hynix talked about its HBM2 plans and how it intends to target its three main markets: HPC & server, network & graphics, and client desktop and laptops. The company confirmed it will offer HBM2 in capacities of up to 32GB, via four stacks with 8 Hi modules, but this configuration will be reserved for server applications.
For the graphics card market, SK Hynix will offer HBM2 with capacities of up to 4GB. This allows GPU makers to put up to four stacks on one interposes, like NVIDIA did with its Pascal GP100, which offers 16GB HBM2.
Then there's also this interesting slide from Samsung, which talks about low-cost HBM for the consumer segment. This type of memory would have a 512-bit memory bus, half that of regular HBM, but it would still offer roughly 200GB/s memory bandwidth per package, versus 256GB/s offered by HBM2. Exact details about how much cheaper low-cost HBM will be are unknown.