Over at the Workplace we've been discussing how to encourage people to tag which country they are from, as it often has a strong impact on our answers. This got us thinking that it would be nice for individual SE sites to be able to encourage, or even require, a certain type of tag anytime a question is created. For example, I imagine that some sites would like to encourage the tagging of a coding language, or an operating system, or a gaming system.
One option would be to add a per-site tagging guidance in the "How to Tag" pop-up during question creation. This could be done for the "How to Ask" and "How to Format" pop-ups as well. Alternatively, there could be a completely separate warning or suggestion of what to tag near the actual "Tags" bar. This would probably be below, near the "suggested tags", and could maybe even be incorporated into it.
There's also the option for a custom required tag set, like in Meta sites. I don't think that would be applicable for our particular situation, but others may find it useful.
Some sites may not even have a recommended tag set, so any new features should be optional to customize.