although extended metadata would be more like a workaround for the lacking possibility to search system tags with an advanced query system
I do not agree: extended metadata would be really useful e.g. for EXIF data or for bibliographic data.
I think that a tag system (even an advanced one) would be really unfit to manage that kind of information.
In any case I proposed an improved search for tags here:
It was something I was thinking from some time and your note gave me the momentum to write it down. In case you are interested, just comment or upvote.
For the extended metadata, I imagine that it is really more complex.
Hope that helps!