Just because it is open source does not me (a) that your changes will be mainlined, (b) that your changes will be even considered or (c) that you will logistically be able to maintain an ongoing fork.

Saying "it's open source so you can just add feature X or change feature B" only works if you are part of some core team or are respected via other means. Github is littered with projects that have lots of Pull Requests that never get pulled.