I understood this to mean slots and bays. Slots for lots of RAM, possible a slot or two or more for m.2 SSD. Slots for graphics cards and other cards.
HDs (and the user could add a shiny optical disk if they so desired). Possibly 4 bays for an internal RAID.
That’s what I think of when I hear modular. Upgradeable CPUs? In my mind, the type of user that really needs to stay current with CPUs has the means to just buy a new computer* — although tinkerers might like this, they don’t really need it.
This is what I expect to see, more or less. It’s not really what I want now or think I will want in 2018. What I want is a sufficiently powerful Mac Mini that is not terribly difficult to add RAM or replace HDs (and with slots for SSD cards). Who knows? Maybe the prices for used cylinder Mac Pros will come down far enough that one of those will be my Mini replacement.
*Assuming Apple refreshes the Pro desktops on a regular basis.