Hello. I have some stake in this issue too.

The proposed patch is ok, but I think that we can go for a more 
ambitious solution here that would require just a little more work, but 
would be a lot more useful.

Maybe lets have a make-file-backend function, the output of which you 
can cons onto project-find-functions, so any file defined like this 
would be just another separate backend.

This function in turn can be used by major mode developers and users to 
implement project backends. If we look at the list of projects supported 
by Projectile 
<https://docs.projectile.mx/projectile/projects.html#file-markers>, we 
can see that this one function is enough already to implement most of 
them. Having such file backends also leaves the question of whether the 
particular file should have a higher priority than VC to the end user, 
depending on where he(or some major mode he's using) adds a particular 
one into the list.

As for whether there should be some kind of default file marked project, 
sure, the user can just remove it if he want to.