I'll start this question with example of the problem I'd like to solve.
I like to tag my apps by category: productivity, development, media, etc. I like to tag my movies by genre: action, drama, porn, comedy, etc.
I like to tag my personal documents by some other criteria: bills, medical, auto-related, etc.
All these tags end up in a big messy list. So when I go to tag another app, I have to scan this list of items irrelevant to current context. Applying "comedy" to apps is invalid action and, in perfect universe, would not be available.
Is there a way to somehow define contexts where certain tags become available while the rest of them disappear? Is there a way to organize the tags other than them all being in on big stupid system-wide global list?