If you go in with the intention of writing an extension rather than a general case fork then it becomes a bit easier. You know from the start that you will reba

If you go in with the intention of writing an extension rather than a general case fork then it becomes a bit easier. You know from the start that you will rebase on top of upstream over and over again, and you should write code taking that into account. Minimal changes to existing code, ideally put your code in separate files and make single line changes to existing code to install hooks. Consider what code and interfaces are stable and what changes a lot.