It seems appropriate to me that some posts can have multiple tags. Like many posts can appropriately be tagged with all three of "bitcoin", "econ", and "news".

There could be a market system of tag usage, where to attach a tag to your post you'd have to pay a fee. This discourages spamming tags to your posts and choosing only the ones that help searchability the most.

I'm not sure who the "owner" of the tags would be, but the current system where you pay for ownership of that tag could be continued.

Just my 2 cents.