I, like many nuts-n-bolts computer/retrocomputer hobbyists, have a long list of projects I occasionally devote brain space to. Some of them are conceivably attainable; many more would theoretically be possible, but only if there were a few hundred of me and an unlimited budget. Among the latter, which I fondly think of as Impractical Pipe Dream Projects (IPDPs -- yes, I have so many as to have developed a nomenclature), is a ridiculous plan to reverse-engineer classic Mac OS and write an improved version "Like Mac OS 8 (and X) Should Have Been". The basic idea is to write a protected, preemptive, hardware-independent OS, able to run normal Mac software, that could take full advantage of later Mac CPU hardware like PPC (...and, reluctantly because it's already been done to death, x86-64) -- but to do it using natural extensions of classic Mac OS software technologies, rather than completely displacing everything with some alien beast based on NextStep and UNIX, like Apple actually did.

I am 100% certain that I have neither the skills nor the resources to actually get anywhere meaningful with such a project, but startlingly, it seems a good starting point in the form of the source code to System 7.1 has semi-recently been leaked onto the net. Naturally, a surprising amount of that code is written directly in 68k assembly language, but it's something.

That's where I would start with any OS development project for vintage Mac hardware -- a system made to run Mac software in a Mac-like way. As pointed out earlier in this thread, we have Macs as opposed to some other platform for (what we, at least, feel are) good reasons. It only makes sense to work with that. Such a plan would also take care of the "But there's no software for it, what's the point?" problem someone else brought up.