I've recently been giving a lot of thought to what I would want in my Harry Potter Fan Cave if money were no object and I could afford such a thing. I can get a bit carried away with my imagination on these things and probably spend entirely too much time in my dream world so yes, the room would definitely have the mirror of Erised in it.
I don't think I'd be crazy enough to buy a castle and want an entirely HP themed castle, that's even a bit beyond my logic, but I am just crazy enough to defintely want an HP themed room inside a beautiful mansion some day. =D
The entrance to my room would have a picture of the "Fat Lady" on the front door and if possible you'd need to recite a password to be able to enter the room.
I would want my room to be on the second level of my home and I'd want to try and design it with the Gryffindor Common room and Harry Potter memoriabila in mind. The house would also have to have a turret built off of the common room so that I could go up to bed in the turret any night I chose to. The room upstairs in the turret would absolutely have a four post bed, an Invisibility cloak, a poster of the Chudely Cannons on the wall and nice window for star gazing.
Back to the common room though. Some things I would want would include three stained glass windows. One with a Stag, one with and Otter and one with a Terrier. The colors of the room would be Gryffindor Common room colors, with several comfortable upholstered red couches and chairs. A large fireplace would be important too. The carpeted floor would have the Gryffindor Lion emblazoned upon it. A card table and a spot to play wizard's chess would be a must.
I would have a chest or cupboard with a sign saying "beware of boggart" hanging on it. There would be several book shelves and they would be filled with as many magic related or fantasy/fiction related books as possible and the Harry Potter books would have a place of honor on the shelves of course.
A memorabilia display would contain items such as the horcruxes (well... save Harry himself), a time turner, a Quaffle, Bludger and Golden Snitch, a Nimbus 2000 and Firebolt, the Marauder's map, a Putter-Outer and a prefects badge among other items. Oh, I forgot to mention my wand (the ONLY HP memorabilia --- well along with HP cufflinks, that I own currently) Above the case would be the sword of Gryffindor.
A small bar just off the common room would be called "Hagrid's Hut" and have Firewhiskey, Pumpkinjuice as well as several of the Harry Potter inspired alchoholic beverages I found on the interweb.
I'm not the kind of person who is big into autograph hunting so I doubt very much that I would try to get my memorabilia autographed by cast members. It's oddball enough of me to imagine an HP fan cave without carrying what would become my obsession to those extremes. >.<
I have other ideas too, and I'm sure if the Lord or Fate sees fit to bless me with untold riches and indulge my fantasy there are things I list here that I would want to keep and ideas I might scratch, but this is just a start and just for fun. I'll return here and share some more of my ideas as they come to me.
Of course, if anyone else has thoughts or ideas on what they would want their HP fan cave to look like, please feel free to post about them.