It's a pretty incredible achievement - some things I would love to see added to it are some way of grouping mappings, or making particular sets of them special - like Gelbooru's mapping, and Danbooru's, and etc. This would open up the door to having transformations between sets of mappings (functors, you could perhaps call them?). For example, if you wanted to use Gelbooru's mappings (they are less strict than Danbooru) with Danbooru's tagging conventions, you could write some mapping of tags to tags (or, more like a mapping from mappings from hashes to tags, as the transformation of a single tag might depend on the presence of other tags), which could transform some of Gelbooru's tags into a set that follows the conventions followed on Danbooru.
Another important open problem seems to be that cryptographic hashing as the only equality metric for files is clearly too strict - thumbnails, minor image transformations (like PNG -> JPG), watermarking, etc, all appear as "different files" to the tagging system. I would love for there to be some way of equating files modulo some other function, like for images there is color histograms, keypoint matching, phash, etc.