I had my thoughts about this a while ago.

The best I came up with was having subtags to a certain tag.
And more thing I'd like to have would be when you are writing a tag to your post, there comes up a selection box, for example, writing-(poetry, story, blog or others + add new option) In each of them you'd see how many posts are there with a tag like this.

It's easy to get lost in those tags.