I propose a tag variant which I think is important, but this is a special tag which cannot stand alone. This tag means the variant of another tag.
Smells like a meta tag to me, and therefore wouldn't make a good tag... It makes no sense on it's own and it arguably means different things to different people (or depending on what other tags are present).
I don't think there's anything fundamentally wrong using tags to indicate that a question/puzzle has aspects of that thing (i.e. your example is fine tagged with sudoku).
I don't think it would be necessary.
My guideline is this: I only make new tags if
- someone might want to ignore all posts of that type
- someone might want to only look at posts of that type
I can't imagine either of those cases occurring for variant.