I finally reviewed MUC Sub a while ago, and unfortunately the details provided are not complete enough in order to make a full independent server implementation of the MUC Sub protocol.
Additionally, no open clients support or, as far as I am aware, are planning to support MUC Sub. There are a bunch of competing standards (Mongoose has "MUC Light" for example). The XSF is mostly focusing on MIX, which is at least being developed through an open vendor-neutral process.
For now we are currently committed to improving MUC, and making it work really really well. We have made good progress in this area, and we continue to do so. This has the advantage of building on a mature protocol and staying compatible with the vast ecosystem that we have today. If the XSF and/or the ecosystem begin to move, obviously this may change.
Implementations of other protocols are possible (and welcomed) as contributed modules.