It is not decided yet to use Matrix, I personally like matrix because they are being peer-to-peer, no single entity dependency or point of failure, seem full featured and only focused on chatting. Would be interested to know, what feature you didn't find in Matrix that you wanted in OS?

Another option I suggested before was, why not create hivemind plugin that will just work with existing users and for Steem users, we have seen many attempts in past with custom_json but tools and standards wasn't ready then, but now I think, almost every full node runs hivemind and with communities it will be even more big part of ecosystem.