Really interesting.
I am still a n00b here, but here is the challenges I expect:
PCB: Don't know if it would be possible, but lets say it would, so then...
Plate: Probably the same as PCB, hard but possible, probably easier than the PCB since theres nothing "in" it. But cutting it with precision after it is produced would be hard. People are talking of lasez cutting with these to get enough precision, so...
Keys: Your design seems to use the same layout with additionnal keys when you increase the size, but the 75% set would fit them all. But now it would not support all the variations that some prefer. It would need to be some kind of one size fits all or something like that.
Case: Can't make a generic case that you could just cut (unless someone is a real genius). So we would probably have to make one for each setup. Still it could be the same design, with variations on the size.
It could be interesting to evaluate the gain of a universal pcb that you can cut over a universal pcb design with 4 variations. Same with plate and case.
Still, I think that once you have a final GH60, making these variations are not that hard (aint diminishing the hard work that is done here, just saying that making a 75% should not be harder than a 60%), but supporting many different design brings challenges and costs, but it also increase the market
In the end, supporting more sizes will probably mean more work to be done, and with all the projects I see running here, that's no small thing. Still having some standard that we could greatly reuse with each project would be awesome, but it's what most of the stuff here is in a certain way.
Anyway, very interesting +++