This is where I would see the "modular" aspect come in...
If modular is primarily center around storage it probably not significantly like these "lego" brick concepts that have been floating around.
There could very well be four M.2 slots, but they would be for Apple-proprietary M.2 SSDs, all controlled by the T2 chip, and you have to install them in pairs...
It is very unlikely Apple is going into some "sold in the store" blade SSD business. If there are secondary storage slots keeping those proprietary storage slots that doesn't almost nothing there for Apple.
First, there is almost no "volume". Entirely optional BTO components don't have a significant base volume. Apple putting a specific blade into every MBP 13" sold is an entirely different economies of scale equation. ( Case in point they have never sold these Apple specific SSD blades before in open market. Why would they start now? )
Second, all of Apple's internal SSD controller work is to incorporate the controller into a wider functionality system on a chip (SoC). Putting a T-series or yet another A-series derivative not only doesn't have scale it would probably cost even more from the solutions already out there. So no volume economies of scale or BOM win here. [ The option of just chucking the now old SSD controllers from the old SSD. .... How much is that going to help to put dated SSDs into the new Mac Pro ??? ]. All of Apple SSD controller work is away from SSD blades [ controller mounted to logic board].
Third, Apple providing "trimforce" means Apple already tacitly admits that folks will use non Apple SSDs. If buying a couple of several, providing a path for that is already there ( with Thunderbolt on the systems is always there anyway ). If Apple sells one SSD with every Mac they already have that volume , revenue, and profits. To throw it on the secondary one(s) borders on ultra ridiculous Scrooge McDuck myopia. They are going to loose more in aborted sales than they'll gain. It would be far more cost effective and cheaper to just spend the money to qualify one/two SSD M.2 supplies and perhaps slaps an Apple marked firmware on them ( like what they did early on. )
Some mix of M.2 (and maybe 2.5" to allow in lower $/GB SSD) is the only sensible path to "more internal capacity". It would be more cost effective for Apple too. ( since not creating some very low volume device in the "copious spare time" they don't seem to have anyway. )
That's how they get you...! ;^p
They only "got you" after you bought the system. There are a significant number of folks who have the budget and want one big pile volume for all of their data. Those folks will buy the very high capacity default SSDs and Apple will live just fine off of that. However, there are also other folks with tighter budgets than Apple's inflated SSD $/GB pricing enables. They'd already be seeing a drop off is prune off the 3.5" and 2.5" crowd if they go that route. Making the customer base gratuitously even smaller won't help the system's ecosystem over the long term.
Four standard 2280 M.2 slots on secondary storage daughtercard with integrated RAID controller
That's probably a punt to Thunderbolt (if there were no slots). Don't really need optional SSD slots where there is some requirement where have to fill more than one at a time. The Mac Pro 2008-2012 had four independent sleds. That would be entirely sufficient for a wide spectrum of use cases . Apple isn't likely going to sell a BTO RAID card with just one SSD on it. Nor a RAID card with the set to JBOD. Far cheaper would be just to put 1-4 M.2 slots on a PCI-e switch ( so they all appear independent. And primarily just get an "added aggregated" capacity uptick from it. Which is the main objective here "more capacity" not "more storage bandwidth speed.". ). [ Even more cheaper if just use the PCH chipset to be that chip.... but I'd suspect they'd want to manage the DMI bandwidth pressure; so hang that switch off of the CPU> ]
If someone wanted to stack that back up into a software RAID ( future version of SoftRAID) they could and Apple doesn't have to do much directly. ( primarily just like the Mac Pro 2009-2012. And they probably won't mind the hackery going away of folks doing redirects off of the "drive sleds" to 3rd party raid. )
For third-party add-ins (Blackmagic Design will partner to provide a 12G SDI 8K daughtercard at launch & BTO)
That makes about zero sense. If Apple is trying for 3rd party, bus connected, add in cards intenrally ...... they don't need a proprietary slot at all. Perhaps it is only a x8 ( or x4 ) PCI-e legacy standard slot ( and nothing but bus power allotted ). And if have one of those and someone has a huge, burning need for a RAID card with M.2 slots on it... ta-da ... they'd have one.
That's RAID card above from "Apple". It would fit just fine an a x4/x8 electrical slot. So Apple doesn't have to compose it for sale. ( or even stock it if they don't want to. )