What if we did what others suggested was the practical limit - 48GB. Then just put 2-3 cards in the system and maybe had a little bridge over a separate bus for them to communicate?
If we're being reasonable and say that you're not using a modern HEDT CPU that costs a couple thousand, the best a consumer botherboard can get right now would be 2x 8x PCIe gen 5 at 32GB/s and one chipset x8 PCIe gen 4 at 16GB/s. I'm not sure if a motherboard like that actually exists but Intel's chipset should allow it; AMD only does x4 to chipset so the third slot is limited by that