A good idea to make one in the country it was filmed and thought up in, but I do agree with the guys above who rightly pointed out that it will not last long. Disney Land, who works on the same sort of principle, is only successful because they change the themes around depending on the films and cartoons that are popular at that time. After about a year or two, this Harry Potter Theme Park will probably die out, or be incredibly unpopular.
Saying that, I would be interested to go, assuming it isn't built in about 5 years time, making the whole build pointless post-films :P
