Were I inclined to not only write Harry Potter fanfic, but to write very lengthy Harry Potter fanfic, I would start from the very beginning of book one and create an alternate canon universe in which Hermione — who has always seemed the only character smart enough to really defeat any dark lords lurking about, and who doesn’t lack the courage needed to go with her intelligence and hard work — were the hero, and in which she spent the seven books working out a proper plan for Voldemort’s defeat. Harry and Ron — who can neither pass their classes nor consistently come up with good schemes of their own without Hermione there to do their homework — would be distant background figures as Hermione surrounds herself with more useful sorts who can actually help her get the job done.
(If someone has already done this — because surely someone has — I’d love to know about it. :-))