I have been thinking of this problem (Finding Images Using Tags) in depth and have came up with a very powerful way to fix it.  

Say for instance I want to tag all of my movie stills with the G/PG/PG-13/R rating that the original movie had.  But if I select one rating, I don't want any others selected (Mutually Exclusive).  How do I set this relationship in a way that makes for a flexible, unified system that will meet everyone’s needs?

Example:  Say that I may want a tag to trigger an event that will allow me to select or deselect other tags on the image.  This would mean that the tag either inherits another tag or excludes another tag.  This maybe a parent, but also maybe a mutually exclusive rule that if I tag an image with tag A another tag B must be selected/deselected.  With a system of Triggers and Events, say event TagNamedA->Select happens I can deselect or select other tags.  "Caffine" tag ensures that the "Caffine Free" tag is deselected.

But why stop with tags!

I could also setup rules for folders, categories and all other metadata.  Example: when an image is "Added" to a folder (which maybe when Digikam scans for new files, or when the user moves a file to a folder) which will trigger that image too have certain tags, or remove other tags.  

As you can see, the power of such a system can expand outwards in every aspect of Digikam,  Categories and many other area's may also be included.  This will make for a great way to manage images and automatically assign/remove tags depending on specific attributes. 

The only issue is that with this flexibility, improper usage may lead to loops or other issues which developers may attempt to detect and prevent given the details of the system. 

Reproducible: Always