I suspect the major innovation they're looking for is usability. I can really only think of a single product that is both modular, and very usable in it's modu

I suspect the major innovation they're looking for is usability. I can really only think of a single product that is both modular, and very usable in it's modularity, and that would be cameras (not the point and shoot variety). You have a few major camera systems to work within, but aside from that variable, body, storage, mount, flash, lenses are modular and exchangeable in a very usable way. When it comes to computing, most IO could be made readily modular, as PCIe is quite stable, and the bandwidth needs are easily exposed. Where the problem comes, however, is making compute and RAM modular. Connecting multiple compute units with multiple RAM elements is a completely different problem than IO expansion, as the devices are much more sensitive to trace length and layout, and much higher bandwidth. If the expected/desired Mac Pro is truly going to be fully modular, then I suspect it will have to be NUMA, and have a compute backplane separate from IO. I'm imagining compute blocks that have RAM plugged directly to CPU that can be connected to the compute backplane (something like QPI, UPI, or InfinityFabric) that also has an IO block attached. One big problem is that these interconnects are proprietary, which means any IO unit plugged into that backplane will only be useful for that brand of processor. The problem with this, however, is you lose access to CPU-exported PCIe bandwidth, which is very valuable (e.g. for GPU links). If you don't want to lose that, than you're back to a compute unit with one point of modularity, the RAM, a second point of modularity, PCIe/USB4, and a third point of modularity, CPU interconnect/backplane. It's probably possible to deliver a design that enables this, but it's a very tough problem, and hasn't yet been solved.