Maybe it's because I watched The Expanse too many times, I'm kinda attracted by their concept of COMM system.
Based on how it was depicted, it's a site-to-site (thus P2P) based system that allows encryption, hop-via-proxy and multi-stream transport (embedding files in video call).
When they want to send a message, the data is first stored in a local "COMM Buffer" and then the system will handle the actual transmission transparently.
The data transceiving can be done in real-time if the participants are near. If not, then it will work similar to how email is exchanged (except the data stream is multimedia, not just text).
How is this relates to AT Protocol (and maybe ActivePub)? Well, AT Protocol is designed to be used in "social network" settings, but "social network" was largely evolved from people forwarding emails etc. I think if you could build a really good COMM protocol that allows people exchange information quickly and efficiently, then it should be fairly easy to add social elements on top of it.
I think the idea of building a "social network protocol" itself is wrong. People get on social media to do things, maybe it's gather info, maybe it's to learn, maybe it's to make contact etc. Maybe smart people should focus on building a protocol to enable all that, rather than just trying to build something that poorly mimics what the big platform has already perfected.
Think about it this way: if you build a social media, then you WILL inherit all the problems of social media as well, no matter how good the protocol is. So maybe just build something else instead then.