I just had an idea which seems like it would be really useful for things like web development, but I have never heard of it being done.
Say I am developing a web project and I want to create a starter project which I could use as a base for individual websites. Normally you would just have to make a copy of the whole starter site and then customize from there. But what I would like to do is have a system where I could just create a blank folder and tell it to magically inherit all the files from the starter site folder with the exception that any files I place in the "extended" folder would overwrite the files from the starter site folder. Or maybe you would have Folder A and Folder B merged into a folder C, where B overwrites everything it clashes with from A. I'm not sure how file deletion would work, or moving sub-folders...
So the benefit of all this is that you could make changes to files higher up in the hierarchy and they would trickle down to the extending/inheriting folders, so long as those files haven't be modified in the inheriting folder.
Anyway, am I crazy or are there ways to do this? I got this idea from things like Wordpress child themes and file referencing in 3D software like Maya, but I haven't seen it done right in the file system.