Another kind of tag hierarchy/relationship
The current system supports only tag synonyms where the tag can have multiple synonyms. If a synonym is used it is automatically replaced with the master tag. Tag synonyms are proposed by users with enough reputation and after receiving 4 votes applied to the system (or the proposal is cancelled after receiving -2 votes). Users cannot propose tag synonyms related to versions. It is the priviledge of moderators.
All these tags are indeed .NET. So I think there should be a separate functionality which will allow connecting these tags to .net. In such case a version specific tag will be counted as the top level tag but still displayed as a version specific. All users who subscribed/favorited to .net will see also all questoins marked only with version specific tags.
The separate problems will probably be where to count upvotes and if tag badges should be awarded for version specific tags but I leave these as open problems for discussion.