Rambus released some preliminary details about the HBM3 graphics memory modules that are expected to hit the market in 2019, at the earliest. With a transfer rate of around 4 Gbps, the HBM3 modules could double the memory bandwidth, making them faster than the GDDR6 modules included in Nvidia's GTX 2000 GPUs.
https://www.notebookcheck.net/HBM3-memory-expected-to-feature-4-Gbps-transfer-rates-and-GDDR6-like-bandwidth.268330.0.html