Wow. I've been telling people for years that one of the big theme parks needs to get on top of building a Harry Potter attraction.
I can't remember who above said it, but I would be a little concerned that the peak of the hype has passed. That seemed to me to be about four years ago. The three-year wait for Harry Potter and the Order of the Phoenix, coupled with the release of the first two films, seemed to create the most hype. From where I'm standing, it seems to have quietened down a bit since then. Which is not to say that it is anywhere near dead, or even dying. I teach in a primary school, and I would say three or four years ago the HP hype even outweighed Disney fervor. Now it's about the same.
The main thing, though, is not how strong the trend is at the moment, but the range of possibilities, and I can't even get my head around them at the moment. I'll give it a try:
Hogwarts Castle: In my opinion they need to try and make this as true as possible, and open up as much of it as they can. Lots of the classrooms could make fun shows or workshops. The traditional magic shows (once cheesy but now juiced-up and cool again) could be set in Charms and Transfiguration classes. Potions could also have some cool options.
Flying on broomsticks would be interesting; either they could do this as a simulator (although these seem to be a bit dated now, a multi-sensory one like Spiderman could still be great) or try to make it like E.T. I admit I'm not sure how this would work.
I just love the idea of the Sorting Hat playing a role. There could be a Sorting Ceremony as an ongoing audience participation show in the Great Hall.
Care of Magical Creatures is another source of great ideas. I know they have the Triceratops Encounter in Jurassic Park; an adaption of this with dragons, hippogriffs or unicorns would be fantastic, especially for children.
The Owlery could have some sort of aviary show. I know there have been several of these in the last few years in UO and WDW.
As far as restaurants go, a Great Hall Banquet thing seems pretty obvious. It would also be easy to theme to different times of the year, as it is in the books. Christmas and Halloween are the most obvious, and between May and July the end of term would be good, between August and Halloween the 'back to school' would also be great.
Hagrid's Hut would be a great place to get snacky things, tasting better than the ones in the book, but appropriately Hagrid-sized!
The possibilities for little shops are endless - magic-related stuff in each of the classrooms, House-related memorabilia (please!) in each House common room, also movie memorabilia would probably sell really well.
Hogsmeade: As far as rides go, a haunted-house type thing for the Shrieking Shack (I know, it's not really haunted!) is most obvious. Also, the concept art suggests it will be winter there (as it's described in the PoA book, and shown in the film, I guess) so winter-themed rides would work.
Food and shops - they're all pretty obvious. Sweets and Treats at Honeydukes, a really cool Three Broomsticks pub, magic tricks at Zonkos.
Forbidden Forest: Any number of creature-related rides and attractions could go here, as well as being a good home for a flying Ford Anglia ride.
Other: Underground cart ride for Gringotts bank. I would queue for hours for this one, if it was authentic.
Anyway, that was just stuff that jumped out at me straight away. I honestly haven't thought about it that much

I have revealed the inner geek, though!
I really hope it is open by 2009, because that is when my next trip is projected for. If we know by summer 2008 that it will be later, I might be convinced to postpone my trip. This new stuff will make my next visit.