The PC-9F front panel can fit on the PC-7H. I will send you more info on that in a PM.
While having many parts (front panels, side panels) available for swapping and switching is a great idea in theory, the issue comes down to logistics. It would be extremely difficult to ensure that all the different part options were able for preferential configurations around the world. There would be shortages of certain parts in some places, and perhaps surpluses of that same part in different places. Therefore even if the logistical part between vendors in places all around the world was smoothed out there would likely be huge leadtimes while people waited to get the side panel they wanted. Plus, either the vendor or end user would have to put it together, unless ordering direct from our HQ. I dont really think that most people would want to build their own chassis. There would be some of course, but perhaps not the mainstream.
Not trying to knock down your idea at all, just giving you mine. :)