Daniel Radcliff (Harry Potter) said that the reason he did that scene nude in that...I think it was a French play, he said that he wanted people to know that he could do something other than Harry Potter. His face is going to be associated with Harry Potter for the rest of his life. People are going to always look at him as being "The Boy Who Lived". The author of the books is kind of in the same situation. She will forever be associated with Harry Potter as well. So, yeah, if she decides to write something else that is in no way related to Harry Potter, her fans will probably not take too kindly to it at first. It won't be Harry Potter. Somewhere in the backs of all of their minds Harry Potter will be what they will be craving more of, especially now that the series is over. They're going to eagerly pick up any book with her name on it hoping for more Harry Potter. And they're going to be disappointed when there isn't any, when the next book and every book after it are all about something completely different.
The good thing for her though is that she has proven that she can write some magnificent stories. So, she really shouldn't have much of a problem writing and selling any others after everyone gets over the Harry Potter craze.