While I agree with the sentiment of this poll, I think the current state of Danbooru is a bit too... ehhhh for this poll to be implemented properly. From what I've seen, trans characters being tagged with their preferred gender is literally based on what the site and the users think. I've seen trans female characters be tagged either as a boy and as a girl (sometimes it changes from post to post). I think the reason Testament's tag change went without a hitch and Bridget caused a stir is because so many people have eyes on Bridget as an otoko no ko icon (which, fair).
There needs to be a fundamental overhaul of the gender tags before anything is done IMO, whether that be making it so it depends on the character's gender or the character's sex (and for the latter, hopefully making the main names of the tags something specified towards sex, like "male" and "female"). However, that in itself would cause a huge mess. so :///
also as a side note on Testament, if Arcsys is to believe, They've always been some flavor of non-binary, and them being male in the older games was a localization thing, but that's a subject for a different day