Would be nice to have a modular board as long as it wasnt expensive to do so. So many times in the past I have simply had to toss out boards that were functional but lacking features such as the motherboards with integrated video and no AGP or PCI Express SLOTS with weak integrated GPUs, as well as boards that only have 2 memory slots yet the system could support up to 8 GB RAM but only 2GB per slot, so that maximum RAM is really only 4GB when if it had 2 more slots it would be 8GB.

Most boards tossed out where lower end system boards from eMachines and Acers that lacked upgrade paths, although I did get burned 13 years ago with buying a HP Pavilion 2.0Ghz Pentium 4 tower that when inspecting the motherboard it had a location for a AGP slot, yet it was never stuffed with an AGP socket to take a gaming video card. Since I needed this system most importantly for gaming and PCI slot video cards were so lacking compared to AGP cards for performance I was forced to take a $300 loss on this selling the computer to someone else who wasnt a gamer. Looking back I probably could have just upgraded the motherboard for $100 moved CPU and RAM over, but at the time I just got rid of it and got a Compaq S6030NX instead for $549 at Radio Shack which came with a Athlon XP 2800+ 2.08Ghz CPU and it had a 8 x AGP slot to accept the Geforce FX 5200 128MB AGP video card for gaming.

http://www.tomshardware.com/news/msi-modular-motherboard,31532.htmlModular Motherboards were already done in the early 90's. The idea was that as technologies changed (ISA, PCI, VL-Bus, etc.) you would swap out parts of the motherboard. Computer Chronicles COVERED ONE product that did this:
here (shortly after the 16 minute mark).

over-modularization simply doesn't work in the longer term. Components become more expensive, there are more failure points (More card-edge or pin connectors to connect all the components) and the interconnections have to be designed with some assumptions going forward, and eventually a new technology will break those assumptions.It's an interesting idea but as far as I can tell it would be impossible from a technical standpoint.  For example, it's hinting at just replacing the IO shield or PCI slot area to customise the interfaces AVAILABLE which totally ignores the fact that it's not the connectors themselves that defines what is available, it's the rest of the motherboard, namely the chipset or other controller chips themselves.  Not to mention that motherboards are now so cheap that something like this would be almost impossible to price competitively.

It me a bit of the NLX form factor (That I coincidentally made a video about a couple of days ago https://www.youtube.com/watch?v=PTlQF35p9_I) where the idea was that motherboards have a standardised card edge connector that goes into a slot on a riser board that provides all the I/O and card connections.  It totally falls flat when new standards come out (e.g. SATA, PCI-E) where the card edge connector would have had to have been updated to carry these signals.Your NLX Video was what I was thinking of with regard to new technologies breaking assumptions. When it was designed there was no way to design it to be forward thinking enough that SATA capability could be added onto it.

Now, my more direct thinking with regards to a modular motherboard was the sort of idea where you would practically build your motherboard out of different components; There would be different "modules" to support different expansion options, Memory, CPU sockets, and I/O. So you'd swap out a DDR3 memory board for a DDR4 memory board; an I/O Board with EIDE support for one that supports SATA; a expansion board with one PCI-E 16x slot for one that has two to support crossfire/SLI, etc. At first glance it sounds like a great way to make a forward-compatible system but the way those components work together don't make them particularly upgradable; SLI/Crossfire would depend on chipset support, which also determines I/O support as well as CPU support, and CPU support determines Memory Support, and so on.