Commenter Paul suggested that a Harry Potter RPG would be limited by the problem of knowing the characters and the world too much, in his words:

1. You’re stuck playing a game where the grandest things that can happen are the books and your characters a left with a feast of crumbs. Harry Potter is facing Voldemort! Can you keep the Dementors from the folk of Hogsmeade while he saves the world?! Or
2. You avoid the Harry Potter setting either in time or location, but these strip the familiar elements from the novels and rob you of the reason you’re playing it in the first place. Or
3. You play Harry and friends, but you already know the plot that you’re playing through

I think this is a similar problem to the kinds of situations you’d run into with, for example, a Dresden game or a LoTR RPG (or Dragonlance, as Paul noted). I’ve got around this in LoTR by choosing option 2), for example – and once ran a LoTR game where the players did 1), in Mordor – they were captured soldiers just trying to escape while the war of the ring continued somewhere far away. There’s no reason to think that the problem couldn’t be surmounted in a Harry Potter RPG.

So here’s some ideas for two different layers of a Harry Potter RPG.

For Younger Children

You could have quite an entertaining little game getting up to hijinks in Hogwarts itself – it’s virtually a sandbox campaign if you want to play it that way, but there are specific inter-house rivalries and shenanigans that can play out against quite a deadly backdrop. I did this for a group of schoolkids I was GMing in Japan, having them start in their school club house and save the town of Matsue from a demon-conjuring older student[2]. You could set up a kind of Ars Magica style of multiple-PC setting, where all the PCs are from the same house, but with different (house-specific) skills, and perhaps with the players having a starting preference. Then, for different challenges in Hogwarts you choose your PCs to match. Maybe there isn’t even a death option, but if something goes horribly wrong you don’t die, you get Dumbledored: one of the teachers turns up and saves you, but then you’re in detention and suffer an xp penalty, or you have to play a different member of your school’s house while you wait for your previous member to get out of detention. Also the goal of some adventures could be mischief against older kids, and you could even define a term or school-year timing process, so that at the end of a fixed number of adventures all the students gain a level; the amount of individual adventuring the kids did in their year partially determines how much they gain from their year’s education (so you tie the adventuring to doing better in school). Thus a good campaign arc also follows the arc of the stories across multiple years; you could have event tables for the summer holidays and for the school as a whole that follow a Make You Kingdom kind of style. This gives the game a more campaign-y, abstracted style, with the players not having to care about getting too bogged down in individual PCs and getting to fully explore the environs of Hogwarts (and maybe it’s a different Hogwarts each time, if enough random tables are used). They can move onto the hard stuff as they get older.

For “Young Adults”

(To be said in a yobby English accent, while dancing[1]). Here the game gets darker and more focussed, with a more intensive character generation process and the assumption that the stories will involve only one PC each, in a more traditional style (we’re making a gateway drug here, remember – we need these kids to grow up and head over to the rest of the RPG world). So they can die or get injured, can carry psychological baggage with them (they’re teenagers, so there should be a lot of psychological baggage tables!) and they can come from multiple houses, with the possibility that they’re working for the interests of their own house as much as the group. This style of gaming can allow for hidden magic, forbidden magic, secret exits to town, wandering monsters in and outside Hogwarts, and the possibility of statting-up and fighting some of the teachers, who of course have their own agenda. It could also allow for post-graduation adventures, and the possibility that the PCs go on missions or quests either during or after their training – basically using Hogwarts, the Ministry of Magic and the strict rules of the Harry Potter world to construct a standard exploration/adventure campaign – with some additional mechanics for teenage angst, and a lot of cool narrative tricks for avoiding death, and for deus ex machina-type GM interventions (via Dumbledore). In fact, student-teacher relations could act as a kind of resource in the game, which would encourage players to hide their PCs shenanigans from teachers.

I reckon that could be a lot of fun. There are fun spells from the books, monsters, lost secrets, there’d be pocket dimensions and monsters and forbidden potions and shagging behind the broomstick sheds – who wouldn’t love it? But in progressing through the system the innocent Harry Potter fans learn to love role-playing, and then at some point they think “ooh, I can do this in a world of my own invention!” and then we have a million new converts to our hobby.

Mwahahahahahaha.

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fn1: gratuitous Young Ones reference!
fn2: looking back on that post, the note I left the students as a clue really should have had a classic English translation of Japanese like “Let’s enjoy summoning a Demon together!!!!”