Peter Siebel has an excellent interview with Donald Knuth in his book Coders at Work. Among other things they go into a bit of detail on the work Knuth did on literate programming with WEB and CWEB. The core idea is that the program is written in a manner that explains itself to you with documentation in a much more natural manner. While this is interesting, what I found a little more intriguing is the manner by which updates in CWEB or WEB were made.
Essentially they were done through changefiles. Which is a list of deltas from what ever version you're on to the latest version. If you apply all those changes then your code is up to date and synced with the latest release.
Normally how updates are done is just wiping the old module and replacing it with the new one, then only time in which that isn't done is if you're working with a particularly large application and re downloading would be expensive.
The beauty of a change file is not that it saves space alone, although that is one nice property. More importantly you can update a file and ignore changes that the user has made. This may seem like a somewhat particular use case, but I think it has some far flung consequences.
Consider the situation in which you're working with a third party library. It almost does exactly what you need but there's a few things here or there that you wish you could change to made the code more specific to your problem. Perhaps you need to specify a different comparator but you want to compare across types, or you want to apply some changes in the new release, but integrate other ones later.
If individual changes were released as changefiles you could have all those benefits, and if you don't care about a specific release and want to apply the new version you can do that too.
What I have in mind is a little more expansive than just forking a project and modifying code. More specifically you ought to be able to take any third party function and modify arbitrary parts of the functions or class and have those changes apply in a specific scope. It could either be a change for a specific file, or for the whole project.
One of the great things about this is that it would mean more cohesion among developers trying to solve the same problem. Instead of dividing their efforts across multiple projects that have subtle differences they could work on the same source and tweak what they needed to change.
This of course will not change the differences between libraries that take a fundamentally difference approach. You would not try to take a library that has made certain architectural choices in its core runtime. In part because that would require an excessive amount of work and any changes made the the original would probably not apply to the other. Similarly, there is an issue here in that some functions and files would rely upon other ones. What happens if you change one, and if you do, have you broken the whole system?
In many ways the git model seems to do a decent job covering some of these use cases. Individuals can make modifications and keep things in sync for the most part. You can sync forks with other upstream branches and so on.
I think the benefit here isn't so much being able to sync things, but rather being free to modify with very little overhead. The result is a bunch of custom tuned libraries that do a really good job with your use case. Installing these custom additions could also be done inline with other code, rather than having to install from a dedicated repo. All in all, the git model does most of this already, and this probably isn't worth pursuing.