Does the borrowing of an author's idea rather than their story constitute plagiarism? Say for instance you read a book and you really like the idea the author came up with but you felt that the story the author told was terrible. You believe you could write a much better book. So you create your own story with your own characters, settings, etc. And it is all based loosely on the idea of the author you just read. Is that plagiarism?

Take for instance Harry Potter. Let's say you like the idea of the magical world Jk Rowling created. But you think you can do better. So you write a book like hers but with your own original ideas. Perhaps you acknowledge that Jk Rowling has heavily influenced your work. Is this alright?