Couple of things where discussions rage on for years and years are whether AP is more JSON-first or rather a Linked Data spec, as mentioned in ActivityPub: A Linked Data spec or JSON spec with Linked Data profile?
There were long discussion threads on the fedi around this theme recently (particularly pinging @hrefna, @jenniferplusplus and @stevebate), and many others now lost in fedi history and link rot unfortunately. Key question is:
Is the linked data based extension mechanism sufficient to specify robust protocol extensions?