I don't think popularity is a problem. The Potter books, by making magic anathema to technology, will have a timeless quality about them. So they will play as well for the next generation as the first, second and third. I expect they will be popular for a very long time.
I think the problem with a Harry Potter hotel is what was mentioned earlier, that a themed hotel is not the business Lowes is in and Universal is not in the hotel business. To do what Disney appears to be doing with Star Wars, you need to be in both businesses, entertainment and lodging. Whether there is a problem with JK Rowling or not is different. But as Universal currently has their lodging logistics set up, I just don't see how they can create an immersive, storyline hotel.