I'm interested by the feature you were looking for.
Have you find a way to do this ?
Category or Tag filter for Popular/Recent/Unread page
On Unread Topic list, We can filter by Categories
It will be better that User can filter by Category or Tag when surfing Popular/Recent/Unread too.
For example: I want to find Popular post of Nodebb Plugin Category.
Same feature here: https://github.com/discourse/discourse/wiki/Basic-Usage-Guide
Update for Unread List. (Can filter by Category)