slrpnk.net already has its own XMPP chat where one’s Lemmy username (e.g. user@slrpnk.net) is one’s XMPP address, and I imagine that other instances could do something similar if they wanted. XMPP is federated, so it doesn’t require any Lemmy-side coding for the federation aspect. For instance-wide chat (visible to all users of the instance), an implementation in XMPP would probably be easier as well, perhaps using some form of the group chat functionality. What does your proposal offer that cannot be done using XMPP?