Well, none really, because the work of ActivityPub wouldn’t be performed in my idea. There wouldn’t be mass copying of content to all servers. And the trade off would be that you wouldn’t be able to follow or see content from another server either (just like modern Twitter and Reddit).
The work around though is that SSO should enable easy cross posting from one server to another, so information/posts would be easily shared between servers. But more importantly would be the commitment to open APIs and building/supporting aggregating or unifying clients, so that from a single interface you can follow and read and reply to content from any server.
In terms of servers and users, the idea is essentially to do the content copying/unifying at the user side rather than the server side.
As far as what protocols are necessary here … basically the internet we have now … which is very much the point.
In simple terms the idea starts from a common praise of the fediverse … that it enables posting between Twitter and Reddit and that it is open. Well, how about we build Reddit and Twitter clones just with FOSS, open APIs and cross posting features?