Backstory: Github is the best for developers to keep track of their code versions and work on projects collaboratively.

The Idea: A github like platform for writers to collaborate and version their content.

Deep Dive: Similar to github you would create a mechanism for writers to create content and sync it with your platform. You may have to build a light weight text editor for writers to use as most may not be technically inclined enough to use a command line interface.

Validation: I've asked a few writer friends what they use for collaboration and some mention google docs with comments, somewhat similar to the functionality you could build with this platform.

Monetization: This would be a SaaS play, charging people a monthly subscription to upload and version their content.