Yeah, there can always be a place for plugins or even extensions in cases where some people want something but the maintainers don't, and no configuration option can be agreed on.

My only point was that an extension would hopefully be a secondary approach balanced by engagement upstream. I could see a useful extension acting as an R&D platform to try ideas that are eventually bound upstream. Or I could see one bundling up ideas that failed to get adopted upstream but still have some passionate community around them and providing a way for people to use good work that can't get up streamed for whatever reason.