I don't quite see it happening, but I agree that it would be great if one could purchase separate compute, storage and graphics modules. A base mac mini could be an all-in-one, not too dissimilar to the outgoing 2014, with HDD and integrated graphics. This would be entry level.
Next tier would be a range of compute-only modules (with RAM) ranging into the i5, i7, i9 quads, hexes, and octo-cores, and on into Xeons similar to used in the iMac Pro. Basic integrated graphics could be onboard all of these too.
These could then connect to external storage modules (Apple or third party), and to eGPU boxes where performace scales as high as you want.
All modules could share a similar footprint and could be stacked (some would be taller than others). Each with a separate cooling system and power supply.
Maybe CPU compute boxes could even be added as slaves to the main CPU module (depending on software), similar to the way a farm of mac minis had been used in the past - but now as a stack on a communicating via TB3.
Far-fetched? Possibly.
Modular, customisable and upgreadeable? Absolutely.
I’d guess that tb is pretty slow for interconnecting cpu’s.
And making modular pieces for mini alone would be way too small for Apple to bother.
But let’s say they make modular ram expansion. They now charge about 3x prices for ram compared to ”free market”. They can’t make expansion cheaper. So how much would they charge for that?