I don't think modular in the Apple world means the ability to just plug in standard components from the PC camp (anymore).
How about a unique case that can accommodate a range of CPU and GPU options in a proprietary form factor and all Apple-blessed. Plus a module that allows you to plug in a standard PCI board or two.
It has been done before - e.g. look up the SGI Octane and o2. I fully expect that kind of concept, certainly not the cheesegrater 2.0.
Man, a modern Apple version of the O2 would be a streamers dream...