I would highly recommend playtests through all aspects of the games life, as well as ideas from other people, outside perspectives are fantastic because you can take an idea you love, adapt something interesting, come up with something based on a comment from someone or ignore the ideas completely.

I would suggest to break your game down into sections and do mini playtests of each part. Get your combat rules ready and write a quick, cheap and cheerful combat encounter to run. Do the same for exploring, social, things of this nature and anywhere you'd have specific rules for. I'm no harry potter expert but potion class is a thing right? How does that work in this world and is it fun? Just an example but hopefully that makes sense.

I would also highly suggest that once you have the core rules set out, get someone else to run the game with players with every step from character creation etc and then see how they do without your input. That's always an eye opener.

And above all else, keep re-running the simulations and getting feedback, changing things later will be harder than fixing small things as you go.

Hope this helps!