Forking would be the key to eliminate the gatekeeper problem, so that for certain topics people could publish their own articles or make changes to existing ones without needing the approval of some opinionated moderator or admin. On the German Wikipedia this problem is so bad that earlier this year we had a really big discussion that even extended from the blogosphere to the major news outlets.

What I mean is that Wikipedia could learn from GitHub how a lot of forks plus easy pull requests can exponentially increase participation and generation of new content while still being able to retain quality.

It's not like someone can work on a feature that later gets merged into the mainline, right?

Why not? You could extend an article or contribute some new articles for a topic, publish the content yourself, and try to get it into the main project. Then on the main site there could be a list of forks, and even a network graph exactly like on GitHub.