If the Fediverse largely realized how the data structure underneath ActivityPub works, a well-defined ontology would be enough to define all business logic.
An activity is a well-defined transformation on an RDF graph. If we treated it exactly that way, many things would be much easier. Most of the complexity we are facing here comes from implementors notorously ignoring the data structure of the fediverse (because they find tables so much more convenient).
Activities could right now already be described as SPARQL Update operations already instead of just English.
The next step could be defining an ontology for defining transformations, so we could encode the operations necessary to conduct the activity in the ontology itself, which would get us as far as building what “crypto” gangsters call “smart contracts”, all without changing ActivityPub itself.