From what I've read, the best fan fictions don't change the character themselves, but rather give them an additional motivating factor, which then influences everything else in a cascading effect. Something like a crush or deep respect or some altered backstory of a character that they respect, rather than going from "Draco Malfoid will be nice rather than mean" you instead make her have a reason to try and not be a total asshole all the time.
Ah, Harry, I am glad to see you awake, my boy. You’ve been asleep for quite a long time. Draco Malfoy? Harry, Malfoid’s always been a woman. Professor Snape black? Me gay? Oh dear, you were having quite a nightmare. Anyways, I have to go and belt Dobbie. 50 points to Gryffindor.