Laptops have unfortunately never really achieved the modularity and ease of modification that PCs have. They are seemingly perpetual slaves to cost, form factor and all the concerns that follow.
(typing this from my Novena)
I have a feeling that modularizing (at least in Ara fashion) cutting-edge GPUs and x86 processors is no small feat. Never say never, but it seems unlikely that form factor could be maintained, let alone heat and interconnect requirements dealt with, when simply boxing up existing laptop or PC hardware.
It's likely that a lot of custom engineering and partnerships would be required beyond simply containerizing existing hardware. On the other hand, the trend toward things like on-die GPUs (and more) is promising, but at present you can't achieve truly top-end performance via this route.
The ideal then, I suppose, would be able to just easily snap together the equivalent of a few GTX 980Ms, sprinkle in a few i7s for good measure, and simply have it all work. Going on a trip and feel like packing light? Just take one of each, casually snap off the rest with little hassle.
Of course, if internet connectivity keeps improving at its current pace, application streaming may obviate the need for all of this, but who knows.
ftp://ftp.panasonic.com/pub/panasonic/business/toughpad/downloads/Panasonic-Toughpad-FZ-M1-Configurations.pdf