+1 to
peer-to-peer messaging, not requiring knowledge of SMTP e-mail addresses.
±0 to
notifications within Nabble (if that's the suggestion); my gut feeling is that this could become messy, in ways that we can't predict. Considering mixtures of privacy, embedding, forced redirection et cetera.
For sanity — aiming to not branch the wish list excessively — I'll move this post to a top level, and add a reference to a related topic.
Regards
Graham