To some extent this can be considered as a follow-up of this question: Increasing chat use, pros, cons and the tour. It more-or-less asks how to help in making chat rooms successful. One of possible reasons why some of the chat rooms fail might be that users are unaware of existence of some chat rooms. (In my answer there I have posted some suggestions which might improve visibility of a room - at least those which I was able to think of.)
Examples of some chat rooms which used to exist or still exist but are only scarcely visited are: Algebraic geometry, Calculus and analysis, Commutative algebra, Complex analysis, Differential geometry, Finite group theory, Functional analysis, General topology, Geometry, Geometry and topology, Linear and abstract algebra, Number theory, Set theory, ... Probably some of those areas are unlikely to generate too much interest, but some of them are probably quite popular (at least judging by the numbers of questions in the corresponding tags).
Apart from that there are a few rooms which are supposed to be study groups or reading groups. (Some of them have already been frozen, a few of them still exist.) And there are also rooms which are kind of "meta"-rooms - not directly related to mathematics but closer to maintenance and moderating this site.
- Would it help to have here on meta some list of specialized chat rooms? (In order to make more users aware of existence of those rooms.)
- Could it be useful to make also meta post where suggestions for new rooms could be collected? (And from voting we could see whether there is interest of other users in those rooms. The same is true about voting in the list suggested in my first point.)
- If we decide to create a post for this purpose, would it be better to have one thread, or should the post with suggestions and the post with existing room be separated?
Perhaps it is worth mentioning that a similar thread exists at MathOverflow: Specialized chat rooms. (Admittedly, neither this thread nor chat on MO in general attract any substantial activity. The only room on MO which currently has many users and non-negligible activity is Homotopy Theory.)