I had this crazy theory that what Apple meant by modular for the Mac Pro was starting off with a Mac Mini-like base. I wonder if this is what that is or something else entirely. Basically you would have a base "box" which is the processor, RAM, and logic board. It would have Intel integrated graphics and an small SSD blade so it could run on it's own. Then you can stack components on top of this: GPU(s), SSDs, HDDs, capture cards and similar components for both video/audio production. It could all connect with a series of Thunderbolt 4 connectors (perhaps a variant that allows the components to stack together like lego bricks. The thing I'm not sure about is how the power supply would work, such as needing a larger one with multiple GPUs. I'm also not sure about whether Thunderbolt 4 would be fast enough for professional, highest-end GPU work. Isn't it supposed to be around 100Gbps? Perhaps the reason it has taken this long is they've been working with Intel on that standard (or building their own?).
It could start with a six core processor and 256GB or maybe 512GB SSD and you built it up from there. Starting at $1499. Add on bits as you like. I'm also not sure if they would allow CPU upgrades. Surely a modular machine would have a RAM access door.