We don’t have plans for anything like this in core at the moment (I’m not sure we’d want to? we tend to prefer asynchronicity, and knowing what everyone’s doing at any given moment is kind of the opposite!)
Anyway, it should be possible in a plugin… we’ve recently added a PresenceChannel API that makes this type of thing easier:
https://github.com/discourse/discourse/commit/31db83527b9b02000f133c782e9e6ab4b4a16659