I'm very happy to see work going into this topic. Hiding metadata is the major part of private communication that's not accounted for by any major chat system. I've recently started studying the matrix.org specification, which seems like the best bet for a next generation chat system, but it doesn't account for metadata privacy. It'd be very difficult to hide metadata while also offering many of the other features of a modern chat system that make it useful and convenient for people (e.g. message history that a new client can sync with later).