The tagging system on Stack Exchange sites supports wildcards like
*unicorn* to follow a set of related tags containing the same word (e.g.
unicorn-horn). This is a very useful feature, but it is not really obvious for a new user.
I suggest to notify users about related tags (containing the same word) when a user adds a new tag to his favorites. A popup would show a few related tags ordered by popularity and allow the user to add them with a single click or to add an appropriate wildcard tag.
The list of related tags could be automatically determined by looking at the most used tags and list tags (used at least
x times) that contain the same word. This doesn't change much, so the job doing that would not have to run often.
The advantage would be a much better discoverability of the wildcard tagging feature. It would also help users to watch the right tags and not have them miss questions they are interested in. It would also raise awareness about related tags that the user might be interested in.
This feature request was inspired by the answers in this meta question about the guitar tag.