The NEXT Mac Pro
In the PC world, modularity is very important because it allows keeping costs down. Pay only for what you need now, and upgrade later. At least that was always the promise, but the reality is frequently more complicated. It´s feasible, and most PC enthusiasts will probably achieve most bang for the buck than any Apple user can, or even be minimal in performance and price. I don't envision the Mac Pro modularity the same way. It will allow professionals to adopt the configuration to their specific needs, and it may even be upgradable in some ways but not cheaply. It can, however, be the modular PC that was dreamed many times in the PC world but that only a company like Apple can deliver.
Modularity seems to be the keyword for what professionals want from the Mac Pro, but Apple was always reluctant to give users access to the inside of their products. That may explain why the 2006 Mac Pro, nicknamed the "cheese grater" by some, is remembered by professionals with affection.
It may resemble a more elaborated tower to a PC user, but it was built with the usual apple attention to detail. From the drive carrier that allowed to insert four 3.5" drives without cables or tools (maybe just a screwdriver) to the way that only the expansion slots and dims are the only visible part of the motherboard. So all the standard, upgradable parts are accessible, but not like an ordinary PC tower where all of the components are visible along with a mess of cables (at least the most common at the time). In typical Apple fashion, it may look like something familiar but its execution it's impeccable, innovative and with a considerable price tag.
There seems to be a lot of voices asking for some updated version of the 2006 Mac Pro, but Apple isn't the type of company that looks back. It was a good design 12 years ago, one can argue that it still is now, but after the 2013 failed redesign a retreat to a safe configuration does not feel like the Apple way. I suspect that Apple will achieve something envisioned and sometimes tried by several PC companies, the genuinely modular PC that can be configured by any user without opening an enclosure or requiring any knowledge of internal components.
One of the better-known attempts at a modular computer in the PC world is the Acer Revo Build (2015/2016). This model has a small base containing a fully functioning PC with a small footprint of 135 x 135mm. Adding additional functionality and power was achieved by buying the extra modules and snap-stack them on top of each other. They were magnetically connected, and the base supported up to 3 additional modules at a time. These could be graphics, audio, mobile storage, and a wireless power bank module. They had some novel features: the wireless power bank could stand atop of all the other and use the Qi standard to charge a compatible device like your smartphone and be removed to use a portable power bank. Neat! There was also a graphics model, a sound module with a stereo speaker and microphones and a portable hard drive module. These had 2TB each, and you could stack three of these on the base for an 8TB of combined storage and also use them as portable hard drives contactable via USB cables.
It's a cute concept, but despite the possible additions, the base module is the real computer. Similar to an Intel NUC with a dual-core Celeron, up to 8GB of RAM, integrated graphics, and 65W power supply, and this is not upgradable. The base module was available for sale, but I'm not sure if any of the expansion modules ever reached the market.
In 2014 at CES, Razer announced Project Christine, another concept of a modular computer, customizable in any configuration by the user without technical knowledge, that despite some internet hype didn't become a product.
“Choose any module on-the-fly in any combination, whether it’s the CPU, memory, graphics card, storage or power supply module, and simply plug it in. The PCI-Express architecture of Project Christine automatically syncs the components"
In 2016, Asus showed Project Avalon, a less ambitious concept than Razer, aimed at more at the PC enthusiast that probably already builds its gaming PC (check the video below).
None of these projects left the concept fase or is commercially available, and it's not surprising. Even forgetting the technical challenges, in the PC world, ruled by OEMs and low cost, a project like any of these would be expensive and probably would serve a tiny market making it unsustainable.
Apple has a significant advantage. Not only has the technical capability to design a genuinely modular computer but maybe more critical, its clients are accustomed to paying a premium for quality products. I can envision a modular Mac Pro but not like the examples from RAZER, ACER or ASUS. The 2013 Mac Pro failed in part because of thermal limitations, and none of the above concepts seems capable of housing the most powerful Mac. I would expect however that every upgrade will be as easy as inserting a hard drive in the 2006 version, if not easier. My other prediction is that all its hardware will be custom and expensive. Sure you may be able to upgrade the graphics but not by just buying an off the shelf graphics card. Apple will manufacture and sell you a custom graphics module, and following this logic any upgrade that you need.