There's substantial demand for a spiritual successor to OS X 10.4/10.6/10.9 that isn't being filled.
The difficult part is replicating the ecosystem of applications. From a technological standpoint I would prefer a modern Linux distribution with KDE or Cinnamon over using 2005-era Mac OS X Tiger, though I’d gladly use a version of Tiger that was retrofitted with a modern Darwin core, modern libraries for security purposes, and an up-to-date web browser. However, Linux does not have Microsoft Office, the Adobe Creative Suite, Preview.app, Keynote, Dictionary.app, and other pleasant Mac apps. The FOSS alternatives are not as polished. Even venerable Office 2008 for Mac feels nicer than LibreOffice, and it’s compatible with the XML-based Office formats.
I guess for now I could run Tiger in an emulator, though admittedly current operating systems serve my needs, even if I prefer the polish of Tiger and Snow Leopard. I get really good performance running PowerPC Tiger inside QEMU on my base M4 Mac Mini and on my Ryzen 9 3900 desktop, definitely not as snappy as running Tiger on actual contemporary PowerPC hardware, but fast enough to actually be productive.
The corporate giants are their own thing, but indie/small Mac devs are among some of the most steadfast early adopters in the software world. They routinely have new releases leveraging whatever new things were added at WWDC in the span of a few weeks or months. Many would be thrilled by a project that promises to bring back what they loved about OS X and would be happy to contribute.
The only problem is that the “it” that needs to be built is a set of frameworks that’s as robust as Cocoa/AppKit is, is flexible with language use (Mac apps have long rolled in C, C++, Ruby, and Python alongside Objective-C or Swift for UI), and isn’t afraid to be opinionated (a single well-supported “happy path” to do something is better than a dozen halfassed ways). That’s really the hardest part.
As for Tiger, on the Ryzen box you might get better performance from a hackintoshed x86 build (especially if you’ve got a spare PCI slot for passing through an old supported GPU for UI acceleration).
Thankfully, macOS Golden Gate is proving to be a decent release so far in my tests of the public betas, but I do yearn for the simplicity (and more power user friendly aspects) of the aforementioned OS X releases.