Well, I think decent worldbuilding is a must. Regardless if you're going the route of completely demolishing and rebuilding the wizarding world from scratch or simply building on the blocks that Rowling set in place, it has to be there in some form or another. Naturally, too many people confuse worldbuilding with infodumps and that's how we get all those tedious chapters chock full of information, delivered in the most off-putting ways. It's best when done gradually, bit by bit. No one ever learns everything there is about something in the span of a few chapters, and they shouldn't. In that same regard, books/school and family libraries should not be the ultimate source of knowledge. Not all knowledge has to be readily or conveniently accessible to anyone in the fic, be they the protagonist or antagonist. Make them work for that knowledge. Make them work and from time to time have them fail in their search. This ties into another important matter for what I want to read in fanfics: the wonder of magic. I'm talking about real wonder. Something that will leave you with both an impression of just how powerful magic can be and how intrinsically mysterious. Don't quantify everything. Don't try to explain everything with some kind of indisputable explanation. Let there be mystery and genuine awe in the way you write about magic. Let readers wonder in delight as they read about various acts of magic. And of course, don't limit yourself to just wand spells. Go beyond the norm. Go beyond the world of witches and wizards. For fuck's sake, there's sentient magical beings in the world already, but barely anyone taps at the potential they hold. Instead, everything seems to revolve around one train of thought and that's boring. Next up is decent characterization. Yes, I can understand that children will be children. I'm not demanding, nor am I expecting that they behave as adults. But as the years in your fic pass on, let the characters show that. Other than just changing the Hogwarts syllabus (or the DADA professor) and occasionally bringing up new sorts of magic they learned about in class, let your characters' growth also act as an indicator of time passing by. You can have one-dimensional antagonists. Nothing wrong with that. Just make sure they're not the focal point of your whole fic, else they'll be easily defeated and even more easily ridiculed and discarded in some kind of pitiful attempt at showing off how powerful, how clever your protagonist is. Hell, one dimensional antagonist is best for the early years. Some little runt (e. g. Malfoy) running his mouth off, trying to pull shit - sure, take care of him easy peasy. Just don't make every single antagonist that follows after the same. The main antagonist (if you have one to begin with) is someone you could possibly see the appeal of - even if you (as the reader) have to step out of your comfort zone for a bit.