Not sure how feasible this would be, but my favourite Reddit app (Joey) has this feature.
It allows a user to "follow" an arbitrary post or comment and receive an notifications when new comments or replies are posted. E.g. "Post 'abc' has 9 new comments" or "Comment 'xyz' has 3 new replies".
Useful for keeping up with active threads which have gotten buried in the feed due to age, as well as cases of "I have that question too, and would love to know the answer if someone responds".
Anyway, thanks Bazsalanszky for an already great app! It is my current most-used Lemmy app.
I originally submitted this as an app feature request, but realized that it could also be implemented into Lemmy itself. Would anyone else find this feature useful?