Intel is planning for PCIe5 in 2021 (skipping on PCIe4), so AMD having gen 5 support in that generation would not be too unexpected. Though with the seperation of IO/compute dies I would not be suprised if PCIe5 would only appear in EPYC/Threadripper components.
I expect that the upcoming Ryzen socket refresh (AM5, which is expected to come with Zen 4/Ryzen 5xxx) to have support for (or even require) DDR5, as by then DDR5 will already be available for a year.
In the server market where the extra bandwidth is actually useful, the transition from PCIe 3 to PCIe 4 is pretty far along. Switches, NICs, SSDs and FPGAs supporting PCIe 4 are all either available or announced, and server-class GPUs should also show up next year. Most of those products have good reason to continue on to PCIe 5 as soon as suitable hosts are on the way.
Server-class GPUs kinda showed up first, or at least at the same time: Radeon Instinct MI50 was the first PCIe 4 GPU ever. (The consumer Radeon VII is the same chip)
I expect that the upcoming Ryzen socket refresh (AM5, which is expected to come with Zen 4/Ryzen 5xxx) to have support for (or even require) DDR5, as by then DDR5 will already be available for a year.