|I'd also like an implication system. Looking at the sorry state of tags related to piercings gives a good feel of where this would help (i.e. pierced_* and *_piercing should imply piercing in pretty much all cases). As an alternative to the proposed system of having the implication automatically add the tags, I'd like to suggest two alternative approaches.|
One approach would be to implement the implications as an edit-time suggestion, sort of like gmail's recipient recommendation. This means that implications that aren't 100% "if P then Q" situations would be fine - when someone edits a post and adds a certain tag, they just get offered a list of other, related tags to add if they want. The major downside to this is that it doesn't automatically add the implied tags to old posts. On the other hand, it doesn't automatically add potentially erroneous tags to old posts where the mistake might not be caught for a while.
Alternatively, rather than doing special logging, treat implied tags like special tags (e.g. rating:safe). So when an implication is processed on an image, rather than cirno adding touhou to the post, it adds something like implied:touhou. Alternatively, since it may be difficult to ignore the "implied:" while searching a separate field or table could be used to store implied tags (haven't looked at gelbooru's DB structure yet, sorry). The advantage of this would be streamlining the validation/normalization of tag implications. Just have a few buttons/links (like the ones next to tags in a search results page) to add, remove, or flag an implied tag. If an implication is resulting in a lot of removes or flags then you know that the tag implication is flawed should be modified or removed.
Anyway, just a thought. Not a critical feature but it would be useful in a number of cases.