There's no point in adopting a spec that doesn't actually tell you what to do, other than to have the official stamp of approval of that spec. Right now, any number of completely incompatible implementations of can call themselves OAuth 2.0, but is that really a success? Is that really what we're shooting for?
At a minimum, I would like the PuSH spec to define *one* way publishers can notify hubs, (`hub.mode=publish&hub.topic=x`) and in order for a hub to be PuSH-compliant it MUST support *at least* that way. Hubs should be welcome to support additional methods, such as the two ways already implemented by two different hubs. (multiple topic URLs https://indiewebcamp.com/how-to-push#Multiple_topic_URLs) by adding additional `hub.url` parameters, and wildcard topic URLs https://indiewebcamp.com/how-to-push#Wildcards)