TAGs is very useful and powerful for helping user discovery contents.
I am thinking about 2 new Tag features which may be useful for many use cases:
(1) Category-specific Tags : when user go to a specific category, we can show a set of Tags relevant to that category only (if we have more than 1000 tags , it's not good to show some unrelevant tags for that category).
(2) Tag navigation: for example - we have 2 tags: one is "New York CIty", another is "Sushi", how do we support the use case like this: user click tag "New York CIty" -- it show lots of topics being tagged with "New York City", but the user want to further narrow down to see all posts being tagged as "Sushi" -- but only those topics/posts also being tagged as "New York City". our current tag navigation seems not support this kind of use case yet.
my 2 cents.
@miluo I love it! Redis supports this kind of mixing for tags too, so we can easily intersect the two distinct sets for "New York" and "Sushi" and see what topics have both tags 
As for #1..... what about "top tags" in a category? I think we can return that too.
@julian for #1, "top tags" in a category - just show top N in case too many tags in that category. that's great. i am wondering how to let user specify a category-specific tag when user post a new Topic.... or maybe only allow admin to pre-define a set of category-specific tags.....
for #2, "NY" tag and "sushi" tag intersect.... the issue i am not clear is when it start and end the tag intersect/combination when user navigate the tags in a session...
@miluo Well, {relative_url}/tag/new-york/sushi would return new york and sushi, removing sushi would just show new york.
@a_5mith cool. but... say i am a user go to a nodebb site today, i cliked tag A, it show a list of topics.... then i click tag B, the issue is how do we know the user want a intersection of A & B , or the user just want to get all topics tagged as "B", indepedent of tag "A".....
@miluo Well in that case, tag A and Tag B would point to the same topic. If Tag B is being displayed when you're browsing through Tag A, then tags A & B are contained in the same topic. Perhaps at the top should be something like Tag A x Tag B x, if you don't want Tag A anymore, close it, and you're left with just tag B. Then you can define a different Tag instead.
@a_5mith maybe use the breadcrum like home-> tags-> A --> B to allow use to change to a new tag navigation session like home->tags->X->B...... when user jump into a different category and click a tag in a topic, that should be a start of another short tag navigation session.....
