There are 1670 questions in the tikz-pgf tag at time of writing. There was a suggestion in chat (and thereabouts) to have some subtags. This "question" is to gather together some ideas.
Two major threads suggest themselves to me:
TikZ/PGF derived packages
There are several packages (or collections of packages) that have been built on top of TikZ/PGF. I don't know them all.
- PGFPlots (the tag pgfplots already exists)
- TKZ suite (tkz-collection, tkz-fct and tkz-graph already exists)
TikZ/PGF common tasks
There are some tasks that crop up quite often.
- Decorating a path
- Manipulating a path (more low-level than decorations)
- Nodes and shapes
Again, I've certainly missed some.
As usual, tags should add information to the question and not simply add a "buzz-word". While it may not be the case that a questioner knows what tags to add, it should be that when the tag is added, the questioner goes "Oh, of course. If I'd known that tag existed I'd've added it."
Please add more suggestions, and comment whether or not you think this is a good idea.
tkzrelated tags already, I took the liberty of editing your post to mention them.