I've been thinking about writing a fanfic that would be a Harry Potter/The Simpsons crossover. Naturally, it'd be called Harry Potter and the Treehouse of Horror.
It all starts when Bart finds a mysterious device in the grass one evening. He shows it to Lisa, who sees the word PLOT written on its side. She urges him not to mess with it: "It's a plot device! You're only encouraging the author!" Bart, truly moved by his sister's concern, laughs mischeviously and flips a switch on the device. Nothing happens. They look around, conclude nothing is going to happen, and go to bed. (Separately. It's not that kind of a fanfic.)
When the school bus pulls up the next day, they don't notice that it says "Hogwarts Elementary". They also don't notice, once they're on the bus, that it levitates a few inches and then glides off to school...
The elementary school's building has not changed, but the faculty is certainly a lot more interesting. (Groundskeeper Willie has been replaced by Rubeus Hagrid, Ms. Krabappel is now Professor McGonagall, etc.) They're teaching a wizarding curriculum now too. Most of the students are the same, but there are a few new faces.
Lisa takes a liking to the new kid, Hermione Granger, but is soon worried for her rank as top student. Bart thinks Divination class "blows", and he's incredibly bored during Muggle Studies, but his attitude changes when he gets to Potions class and realizes they'll be making stink bombs.
After school, Bart and Lisa see a Quidditch practice on the athletic field and are fascinated. They've never seen this before, but some of the players are familiar to us: Jimbo and Kearney are Beaters (and, might I add, REALLY enjoying it), Nelson is the Keeper, and Harry Potter, of course, is the Seeker. They watch the practice, and after it's over Lisa drags Bart, running, to Harry's side. She looks up at him in awe. Bart says "I'm Bart Simpson, who the hell are you?" Harry is puzzled at first, then genuinely delighted to meet someone who doesn't know his legendary exploits and wouldn't give a damn anyway.
As they walk away from the field, Harry mentions to Hagrid that the new "Burns-o-turf" they're playing on has caused seven twisted ankles, and should be replaced by real grass. Hagrid mutters, "'agrid hears yeh. 'agrid don't care."
Cut to commercial. In the second half of the episode, Lisa struggles to get things back to normal (well, except when Harry's around; then she just sighs happily). Her efforts go nowhere until she consults Principal Albus Dumbledore, who says, "Why, my dear, you have had the power to go home whenever you wanted. Just click your heels together three times, and then stomp on the plot device until it breaks." She does so. Roll credits.