I guess it doesn't even need to be XMPP, specifically. But, some open standard and some level of interoperability would be nice. In googling I came upon matrix.org, which also seems promising, but has the same problem XMPP has of not having great clients (though I also found Kaiwa, which looks like a pretty good XMPP client with Slack-like features). Maybe one of these open source projects will formalize their protocol, and we can all move forward on interop with that.