I think it'd be really cool to have a way to group tags in drop down menus & such! I often use tags to remind myself whether or not a fantasy book has e.g. fae or shapeshifters or mermaids, but those are in very different places in the alphabetical tag order so I have to scroll through the whole list. If I had a group, I could expand it, or go to a sublist, or something like that & see all of them in one place.

I also think this would appeal to people who have much more extensive tagging systems, like Jaina a couple of posts down with her ""author-nationality-" or "set-in-" or "on-tbr-"" tags.

And it might make an easy way to set up a custom graph - just pick that particular tag group!