I think in a (specifically bbc) sherlock au where john is female, she would have long hair in a bun at the start (standard military style, allowing sherlock to still make the deduction) but the series should lean into the idea that john hasn’t quite figured out who she is now that she isn’t an army doctor, and once she starts solving crimes with sherlock and starts recovering from the depression (that john is shown to have) and becomes more comfortable in her new identity then she should cut her hair, either to a bob or shorter and more masc depending on how butch you want to make her. female sherlock has very long hair (getting a hair cut takes vital time away from the work, john) that she never gives any attention to apart from getting it out of her face. unless it’s for a case, at which point she can create beautiful, intricate braids and updos that make john do a double take because last week i saw you tie your hair back with a latex glove and you hadn’t washed it in six days what do you mean you know how to do perfect 1940s victory rolls
