Well, in such a fanfic, Marty might have to deal with a whole new set of social experiences. For example, he'd probably have to get used to different clothing styles and how society treats girls. He could also form new friendships with other female characters, and might face different kinds of challenges in school or in his adventures that he didn't experience as a boy.
His character would likely become more empathetic. Since he's now in a female body, he might start to understand the struggles that girls face in that world. This could make him more sensitive to the feelings of others.
A third preference could be his excitement about becoming a father in the fanfiction. He could be seen shopping for baby clothes way in advance, daydreaming about what the baby will be like, and sharing his joy with his friends and family.