As a true Harry Potter fan, you've obviously read all of the books and watched all of the movies -- several times. Although you may own the books and the movies, there are a few things that every HP fanatic just needs.
Back-Up Copies of the Movies and Books
If you're serious about Harry Potter, you definitely need a back-up copies of the movies and books. You never know when a DVD is going to break or a friend neglects to return a book. Not to mention that those of us that have first editions of the HP books want to keep them in mint condition. Having back-up books can give you the books to read while keeping your first editions safe.
A Hogwarts Acceptance Letter
Yes, ladies and gents, even us Muggles can obtain a Hogwarts acceptance letter for a small charge. Show off your acceptance letter by framing it and hanging it on your wall or maybe you're considering this as a gift for someone else. Regardless of your reasons for purchasing, just do it. It will be the most cherished piece of Harry Potter memorabilia. If you really want to get into the spirit Harry and Ginny's wedding invitation and a ticket for the Hogwarts express are also available.
Something else that every HP fan needs is a wand. After all, what is a wizard without one? You can always browse the internet for an original wand; however, if you want to feel more like a witch or wizard from Harry Potter, you could always purchase a replica wand. The cool thing about the replica wants is that a.) they come in a fancy box just like they did at Ollivander's and b.) the detail put into them is so much like the wands from the movies that you really will feel like a proper witch or wizard.
A Marauder's Map
What is the magical mischief-maker without a copy of the marauder's map? Any HP fan would be stoked about this map because of the detail put into it. It is even made of actual parchment paper. You can even purchase a display case for your marauder's map so you can show off your map to your friends.
Merch With Your House Crest on It
Anyone who is a regular on Pottermore has probably taken the time to get themselves sorted. Of course, being a proud member of your house, you probably want to get some merchandise with your house crest on it. From shirts to robes to scarves, no matter what you're wearing you will display your house spirit proudly while piquing the curiosity of fellow HP fans.