Ai-to-Yukai said:
As a suggestion, it could be as relatively simple as requiring a habit's veil and a long_dress with long_sleeves. As there are nuns with dresses other than black (blue and white being prominent), I don't think a black dress is required.
Definitely agree on long dress with long sleeves - no cut-offs, no cut-outs, no side slits, etc - but I was personally thinking on being loose on requiring a habit or veil for reasons mentioned already. When I was tagging the tag, I did concentrate for the most part on black dresses since that is the most common color, but white and blue would definitely be valid candidates as well.
But we can't have the requirements be too simple - I feel that if there's only one or two elements added to the basic design, it's ultimately fine (so Sister Cleaire's zipper isn't too much of a problem, same with simple designs added to the habit proper, as seen with Double as one example), but once the elements added detract too much from the design, then it shouldn't count. We can look to Toaru Majutsu no Index as an example here. The Agnese Sanctis' nun attire, or at least the variant with a longer wholly black dress, would qualify under leniency rules in my opinion. The cross doesn't detract too much from the basic nun design, and the slit on the side showing the underdress complements rather than detracts from the traditional nun aesthetic (ex. post #2468973). In contrast, Index's nun attire in my opinion detracts too much from the traditional design, looking more akin to a fantasy mage (ex. post #3144518, post #1984080). Specter is a similar example of this. An emphasis on the basics of the design is crucial.
Additionally, I think a concerted effort to keep nuns with lingerie out of the tag would also be important. I imagine that there is some truth to, say, nuns in the past wearing garter straps, thighhighs, and other assorted undergarments, but these days they are used as a means of overtly sexualizing the design, especially depending on the type and style of the undergarment, and the perspective taken to depict it. For reasons like that, some of Sister Cleaire's posts don't qualify in my eyes (ex. post #4300317 [E]), and similar posts that are in the nun tag (though thankfully the vast majority of such posts also have side slits, so they're already out) .