John, this sounds great!
Most of my recent experience has been as a WebHook provider (at Pusher). I'm very happy to share all my thoughts on the implementation of WebHook sending at scale, making WebHooks debuggable for developers, retrying, dealing with badly behaved receivers, when to notify on failure, etc.
I'd like to see stronger guidance about the format of WebHooks, in order to make adopting WebHooks easier for both providers and consumers. I would say there is a lot of consensus already, but the lack of guidance means that every implementer needs to expend unnecessary thought on what are, at the end of the day, fairly unimportant implementation details; it's also harder to share code than it could be.
Subscriptions / RestHooks is another area that would massively benefit from assembling current practices together, listing the use cases, pros and cons, and making some recommendations. There are lots of options (hence indecision), but I'm sure we can all get behind the vision of a future where this is just the way things work!
Martyn