Just as we have separate tags for halakha and minhag, I propose a separate tag for minhag-theory parallel to halakha-theory.
This would be for "questions about theoretical and general concepts of minhag".
I was inspired by this question: Allowed to change Tefillin minhag? which doesn't merely address a particular minhag, but rather addresses the larger issue of when minhag can be changed.
NOTE: one can suggest that the tag is appropriate, even if one thinks that the aforementioned question doesn't warrant it.