In which Harry brews a potion prior to returning to the Dursleys, the purpose of which is to make his stay more pleasant. The effect of this potion is to turn the Dursleys into small furry animals of your choice, which he proceeds to look after to the best of his ability. Bonus points if he manages to get Vernon and Dudley to lose weight.
What I would like to see:
Harry paying unusual levels of attention in Potions class prior to brewing the potion, and actually managing to accrue at least a meagre amount of points for his efforts.
Snape spots Harry doing Potions research when he could be out on the grounds enjoying the sunshine.
Fred and George tutor Harry in the ways of prank potions - boosting his overall understanding of potion preparation in the process.
Use of Basilisk ingredients, and possibly owl order of others - to avoid having to steal from Snape. Bonus points for potions lab in the Chamber of Secrets.
Harry talks to the pet Dursleys and cares for them as he would have wished them to care for him - seeing as he has rendered them virtually helpless.
Harry somehow managing to bond with Dudley in some way or another.
Snape tries to figure out what Harry has been getting up to, and finds in one of the library books a piece of parchment in place of a bookmark that bears the notes Harry took in his first Potions class when he was accused of not paying attention.
Harry has the evidence of his mistreatment at the hands of the Dursleys prepared and ready in case of accusations (degree of mistreatment up to author to decide).
The story takes place any time after first year, but preferably soon after, such as second or third year. Harry has been told that he must continue to live where his mother's blood dwells.
Snape finds out. Preferably after the fact.
What I would rather not see:
Broken Harry, as he'd be unlikely to pull this off.
Excessively Cruel or Disinterested Snape.
Excessively Fluffy Snape.
Nothing changing between Snape and Harry.
The potion wearing off on its own.