xmpp doesn't really have a core - even the messages are more or less an extension. The whole point of xmpp is "each to his own extend", so your "missing features" are in reality "nice addons that most clients have but if you'd like to bring your own homebrew that's fine too" which is how I think more networks should operate. Have the features, but gracefully fall back when the client's don't support them.

The core is deliberately small and is meant to be built upon and not be a opionated and monolithic. Such a protocol will become obsolete rather quickly if standardized.

The features you mentioned are not just possible in theory, but have been implemented by more modern clients.