I'm not sure if you read girl!John fics, but this Girl!John/Mycroft is very cute. :-)
A Covert Courtship (http://sherlockbbc-fic.livejournal.com/2262.html?thread=2181846#t2181846)
Summary:After the first time, Mycroft Holmes’ offering her money to spy on his brother becomes a bit of a tradition. Once a month, Dr. Watson gets dragged into a car and taken to somewhere quiet and out of the way. She’s taken to guessing where she’ll end up next, but since it’s Mycroft she hasn’t actually been right yet.
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A Covert Courtship (http://sherlockbbc-fic.livejournal.com/2262.html?thread=2181846#t2181846)
Summary:After the first time, Mycroft Holmes’ offering her money to spy on his brother becomes a bit of a tradition. Once a month, Dr. Watson gets dragged into a car and taken to somewhere quiet and out of the way. She’s taken to guessing where she’ll end up next, but since it’s Mycroft she hasn’t actually been right yet.