That would be nice. In practice, not even ActivityPub as the underlying protocol is standardized enough to ensure interoperability between the microblogging, threaded conversations, videos, etc. As far as I understand, it's pretty minimal and even voting etc isn't as standardized as it needed to be. So I don't have much hope for another protocol being that well-defined and agreed upon, if we don't even have that.

That being said... ActivityPub defines server to server and client to server communication. I think a good way to tacke this is do away with extra Lemmy, Piefed, Mastodon and Peertube clients/apps, and have all the apps speak ActivityPub with the servers/instances. That's already implemented on the server side. It'd do away with implementing any extra APIs. And make any app compatible with any Fediverse project. But we need a new ActivityPub protocol revision for that. Well-defined and with quite some extras. compared to what we have now. And everyone needs to agree on this and implement it. But in my eyes that would solve a lot of issues that are currently slowing down the Fediverse.