Everything below is rumor and speculation. One of the rumors is that the next-gen Radeon cards may be much higher-end than the current Navi-based models, and that they may feature the new HBM2E memory. Depending on the memory configuration, this could result in cards with up to 920GB/s of memory bandwidth.
Here are some takeaways from TweakTown:
New Radeon card on the way RDNA 2 architecture Navi 10 successor (probably Navi 20) Ray tracing support Not GDDR6, possibly HBM2E HBM2E = revised standard, 8-Hi stack 8-Hi stack offers 920GB/sec up to insane 1.64TB/sec memory bandwidth Memory bandwidth = GOOD Crazy high-end will be server/HPC But new high-end Radeon could be a BEAST ETA = 2H 2020