A lot of the discussion about improving the #fediverse focuses on server-to-server federation (S2S). But that's only half of what ActivityPub standardises. I'm thinking it's more urgent to extend and improve the server-to-client (S2C) half of the AP spec.
Most fediverse software only supports the Mastodon C2S API. That's only going to change if the AP S2C API solves problems the Mastodon one doesn't or can't.