i think it would be better to put some general prototyping space on the full board so that f.ex. you could solder a SN75176BP in there (RS232 to RS485 converter) because it really seems like a thing just a handfull of people would use then
That's an interesting concept. The challenge is finding controller components having a similar pinout so that designing and producing a universal PCB is more cost effective than designing several single purpose ones. The solution that manufacturers often opt for is to design a carrier board with 'standard' mezzanine modules. All the distinct features are packed into those mezzanine modules.
An area where there is actually a void at this time, and a potential of growth because of its usefulness is cellular controllers for data communication. Up to this day, I found two manufacturers having huge family lines because of all the cellular frequencies around the world. So, the idea of just soldering a relevant parts kit on a 'universal' PCB to get cellular services is very appealing to me.