You're looking to the Harry Potter series for a good mythos? There's your problem. That's why I used to hate it, especially after seeing a universe like Halo or Star Wars or Warcraft and stuff that are much more detailed.
Harry Potter isn't something to look to for an epic fantasy world. It's something to look to for good characters/development and an interesting coming of age/good vs evil story.
Whatever you do, just don't try to look into the details for the universe... they don't make
sense.