The upcoming AMD RDNA 3 Navi 31-based RX 7900 XT will be based on a 5 nm + 6 nm MCM and could use a 256-bit memory bus but stick to GDDR5 VRAM. Navi 33 (RX 7700 XT) will be a 6 nm monolithic die with 128-bit GDDR6 VRAM and offer more performance than the current RX 6900 XT. The Nvidia Lovelace AD102 RTX 4090 will continue to use 384-bit GDDR6X memory and have GPU clocks greater than 2.2 GHz.
https://www.notebookcheck.net/AMD-RDNA-3-Navi-31-MCM-to-feature-256-bit-GDDR6-memory-Navi-33-expected-to-outperform-RX-6900-XT-at-US-450-Nvidia-RTX-4090-AD102-to-use-384-bit-GDDR6X.562590.0.html