I would very much love tag containers only to avoid visual clutter in my tag pane, but I totally understand the storage problem described by WhiteNoise.
Lise also touched on a way this ties into another concept, the ability to use single tags in different parent/child combinations. I think this presents an alternative answer to the organization problem.
This is more an issue with how it’s handled in search than storage. If tag entry in notes remained the same, but search for a single string returned any matched tag segment tagging would essentially become a one-to-many approach. This could simplify tag entry, tag organization, and tag based search.
Consider the following structure:
If I want to see all #child tags I have to do a combined search for 7 different tags. What I propose is in this case a search for #child should return results with any of those tags. Where a search for #sister or #cousin would only return 2.
The default click action could still search the full tag and therefore exclude other tags, keeping the essentially the same functionality, but adding options to use tags more flexibly.