Well, you could have thresholds, like show if more than 20 users are reading/viewing a topic. This way it would only show those where something is “really going on”. We had this on our SMF forum for categories and I think it was very useful… when users came to our forum they would immediately see a lot of users concentrated on a certain category and realise that something was happening there.
I mean, even out-of-the-box discourse has a similar concept. The columns with the replies and views are also coloured differently to highlight more viewed/participated topics. Here it would almost be the same thing, but with a more “real-time” feeling (and more appealing and useful, in my opinion).