Initially I wanted to explore whether such a WordPress plugin would be popular. Then I realised the topic is interesting per-se, so intentionally skipped talking about the plugin.
What I mean is something like this: Discourse Who's Online Plugin, however, with more configuration and layouts, and embeddable in WordPress, e.g. via a widget or shortcode.
So, putting plugins aside, I was interested in the use-cases, whether anyone thinks it is useful at all.