My main idea for this was that as the character you are given a timetable, and to make it more fun you can attend these lessons any time (other wise it will become to boring).
In the lessons you will be taught spells or potions, you will learn these through mini-games and/or challenges.

The mini-games, for potions, e.g. You will arrive in lesson and be told all the ingredients you will need, you will then collect the ingredients from the store cupboard(although i think i might make it so you explore the grounds to find them), you then return to your desk with them and add them in the correct order (this is a maybe, as again it might become boring), then once this is done you will hand over the potion and it will be graded, once you have learnt the potion you can then make it anytime in i am thinking moaning myrtles bathroom , like in the chamber of secrets book, this will allow the character to procure the ingredients there-selve and produce the potion any time.

If you want more details feel free to ask.

Sorry that was a grammar error. The game will be a mix between an RPG/GTA style free roaming and other HP games

The main thing i want to concentrate on is the free roaming, being able to go to all locations in the book but still keep it relevant to the story, i also want to make the lessons a very good and unique feature, and finally the spell casting system, which will most likely be the hardest as i want it continuous but i can only think of a reasonable turn-based system.

I want quidditch to be a good mini-game, it will allow you to fly all around the pitch high and low, and will have real playing, i may also try and make it an online experience.

Maybe, i will have to see when i start coding.

Again, this will depend on how fun it is, i will be experimenting with the ideas, playing them in-game.

I have a fairly good idea how it will all work and look.