Memory makers are expected to ship GDDR6 in early 2018. AMD made a big commitment to HBM but that memory technology is simply too expensive to serve all segments of the market. AMD did not adopt GDDR5X but it's believed that the company could use GDDR6 for mainstream and lower-end video cards.
The GDDR6 specification allows for up to 1024GB/s memory bandwidth with 16GHz GDDR6 and a 512-bit memory controller. That is the same as what's possible with HBM2 running at 1000MHz with a 4096-bit interface.