As many, many GMs in OSR/NSR circles report swapping out bits from one system to use with another, I think it only reasonable that there are lots of folks who would love a compendium or compendiums that provide multiple alternatives for specific sub-systems.
I’m certainly not the first person to think such would be useful. I think my little useful bit to add to the idea is that such a compendium would be most useful if it provided a Creative Commons that allows for copypasta usage for any purpose. As in “You like this? Copy these word straight into your manuscript” as an expected usage. Shoot, provide text downloads, even (though some pricing to help with hosting and bandwidth costs).
As I find myself sketching out such alternative sub-systems regularly, I think I could certainly contribute to such a project. I’m currently hashing out an alternative combat approach for OSE/OSRIC that involves a tick system using weapon speeds and casting times. I don’t really expect to use it in any of my systems–at least, any time soon–and would be happy to see it used by others. My overflow of usable ideas isn’t constrained to combat systems, either, so I’d be happy to contribute to collections of other sub-systems, too.
If I can find a couple of other people interested in such collections, I might start on a layout bible so they all appear consistently. I enjoy making master pages in Affinity Publisher, and I’m happy to share those, too. Let’s get more people creating their best systems using ideas from wherever they can find them.