In my opinion, xmpp kinda fits perfectly? It was used in multiple platforms before (google, facebook, Gaia online, and gosh the list goes on, every platform 2005-2013???)

Why wouldn't it be a decent solution for the fediverse software to pair with it, just like other networks did before?? Fediverse accounts already have the schema (user)@(domain.tld)

I understand Matrix needs more storage than xmpp, but I think either one could work.