Wonderful!
I’m speculating a little but: as tags are hierarchical, I’d say a drag and drop interface to manage and organise them (like what @Canapin mocked up for categories) could be interesting.
Or, maybe lighter: how about the ability to hierarchise tags in a big textarea (a bit like the tables of settings I’ve seen here and there in the interface). Of course there is certainly a thousand ways this would break, so it might not be a great idea. Actually just remembered there was the csv export/import already, how about « edit in browser » in that format, to start with? Would spare us the steps of downloading, opening in editor (oh hai Numbers/Excel, forgot I had better edit in my text editor), saving, uploading…
The need underlying these two ideas is: give me a more visual space where I can at the same time see all (most of) the existing tags and how they’re related to one another, and change the structure/tags and who knows, maybe some basic properties.
Does this make sense, or resonate with anybody?