A lightweight homeserver. If lightweight enough (which remains to be demonstrated in practice of course), I don't see a meaningful difference from a monolith design.

I see no reason to think P2P isn't a goal of Matrix either; the reason it's not P2P now is that federation was easier, but the long-term game certainly looks to be P2P with work on cryptographic, portable identities and Bluetooth mesh networking in the form of Pinecone. I've also seen arathorn confirm this on HN.