According to this reddit post which teases the LinkedIn page of an so called AMD technical engineer – it is saying they’re working on a GDDR6 memory controller already.
GDDR6 will see memory bandwidth cranked up to 16Gbps (as mentioned in the picture), up from the 11Gbps used on the GeForce GTX 1080 Ti with its GDDR5X memory tech. 16Gbps will blow away even HBM2, which is fairly useless for gamers. NVIDIA’s next-gen Ampere GPU architecture should feature GDDR6, too.
Since the AMD news are a rumor, I can’t say it for sure but it would make sense to keep up with nVidia. Gamers usually not need HBM2 because there are less benefits in performance.
Currently nVidia kicks AMD’s ass with the GDDR5 & GDDR5X interface easily without the need to use the more expensive HBM2 modules.