I've been thinking about how to add support for tag synonyms lately. A slightly technical discussion can be read here: http://trac.donmai.us/trac/ticket/78
The gist of it is automatically rename a synonym to a preferred tag. This wouldn't change how you search or tag a post, everything would be automated behind the scenes. A question would be, who should be able to add new synonyms, and are there unseen issues, or perhaps better ideas?
Suppose everyone has the ability to add synonyms, could that lead to a situation where a vandal merges two popular tags (say, "pantsu" and "oppai") and ends up making the less-used one obsolete (in this case "oppai") and thus removing it completely?
If such changes could easily be reverted, then I don't see why everyone shouldn't be able to merge synonyms. For example, I could've used it when merging "lucy" and "ruuko" tags, but instead I retagged two pictures.
I think one of the problems with tagging on danbooru as it is right now is the unfortunate (but understandable) removal of the "tag suggestions/search" feature - It allowed for easy tag lookup so you A) knew you tagged your images with tags that actually existed B) the spelling was thus always correct C) it was easy to search/checkup if there existed any additional info/element tags that would fit your image by looking elements contained within (eg, see if there's a 'tighhighs'-tag & add that too to that image of a schoolgirl you posted).
It's not really a problem for people who've been here around for a longer time, but new posters are kind of in the blue when they first arrive on how to tag their stuff, I'd say - Relatively looking the amount of poorly tagged images has increased by quite a bit as of late. Perhaps write a more descriptive tagging 101 -guide to the tag-section in the wiki as a start (then again, how many people actually read it? :p)?
As for synonyms, it will of course help to automatically clear up the most obvious in mis-tagging (some sort of ENG-JP converter ought to be the most obvious thing to implent first?). Public synonyms without any sort of sound like a bad idea though, too much risk for abuse if you ask me. Leaves the options of allowing the synonym/tag linking only to certain people (more work for them) or device some sort of control scheme (Synonym must pass a vote before being accepted / public can suggest synonyms but admins/mods make approval, or something?).
Also, synonyms still unfortunetly can't counteract the biggest problem with a tag-based system, which is pure and simply poor/lazy tagging practice in itself (eg, tagging an image with just a tagme" or a series name), but I guess there's nothing one can do to fix that =/
Although technically it wouldn't scale, I think restricting synonym editing to moderators would be feasible, since there only seem to be a few we need to keep track of.
Tag suggestions unfortunately had to be removed because it just killed the server. I think the complexity was exponential, per tag. Buffered got it to a low constant but it was still slow.
I've been thinking of caching suggestions as an extra text field for tags. Users, if they wanted, could refresh these against the current database, and the new suggestions would be cached. So they'd be a little out of sync, but it'd still be a lot better than a dozen requests per second asking for suggestions.
I would think at a minimum you'd want to restrict synonyms to registered and logged in users to help with vandalism, that might be more scalable. Maybe have a flag on a user that defaults to allowing them to add synonyms and if they abuse it change the flag to deny them to do it any further. Then again that'd still require moderators, maybe an automated thing, if you have 3-4 synonyms rolled back your flag is set to not allow and you have to ask to get it reset (and explain why you should get it back).
Or maybe something a bit more complex, perhaps an explanation FAQ on synonyms and a test, where you have to answer questions that the FAQ explains to get the tag set to allow you to set synonyms. That'd probably require more coding up front but might be more mangeable/scalable in the long run.
I know there are some tags that I have no CLUE what they mean and some of those don't have definitions yet. Some I can guess based on content though. Having synonyms that allowed linking an English term to the Japanese term would really be great for those situations. :)