Add some formal relations to tags, to help new users tag their questions more precisely
117963
Select and Display 'Suggested Tags' for All Posts Based on Related Questions (or other logic)
133971
Let's have an explicit triage system for questions from new users
100090
Why don't we add a question with the same tag on multiple Stack Exchange sites?
156434
Tagging multiple questions at the same time?
96777
Ways to give users some specific education about question quality and topicality
85415
Filtering questions based on my Favorite Tags
108865
Please include the logical operators for tags browsing in Transifex
4088
How to form this tag question – as a positive tag question, or a negative one?
81564
Should questions be tagged with both a tag and a "sub-tag"?
35170
Creating a new tag before you have enough rep
84478
Do you really want to create a new tag?
66219
Give Tag synonyms their own review queue
106826
Putting things in  tags allows one to get outside the question border
95586
Have "Interesting Tags" that are combinations of tags
39117
is there a way to request a new tag be created?
125928
How does Stack Overflow suggest related tags?
77231
Span Stack Exchange sites for tags, searches, and related questions