There are no tag trees because it would be too complicated.
However, there is one place where specifying a bit of hierarchy is urgently needed. This is code formatting.
I thought recently syntax highlighter to be intelligently detecting the language, but it proved to be wrong. Syntax highlighter has no intelligence, it just uses tags to detect what formatting to use.
It means for example, that JavaScript code won't be highlighted as JavaScript in dojo tag, because there's no language named "Dojo". I must add dojo tag to my question to allow proper syntax highlighting, even if question is so dojo-specific, that javascript tag wouldn't be recommended otherwise.
I propose there should be possible to say that the tag X is the subset of language Y, therefore automatically enabling proper syntax highlighting without adding surplus tags.