In my opinion, you should always ask the author first. Having your changes accessible from the primary repository is much easier and better for everyone, and th

In my opinion, you should always ask the author first. Having your changes accessible from the primary repository is much easier and better for everyone, and there is a good chance the author is happy to accept your PRs. On the other hand, they might not agree with your proposal or might want to make significant changes to the design, at which point it becomes a personal tradeoff for you.