Some authors invent amazing new worlds and rich characters unlike anything readers have ever seen. L. Frank Baum, J. K. Rowling, J. R. R. Tolkein -- these are the greats. But it's not easy to invent an amazing new world; just ask any other author. You don't need to come up with something completely new to be a hit. Some authors have done very, very well with an un-original idea. Let us not forget that Fifty Shades of Grey started out as Twilight fan fiction. You might be inspired by something old, and end up creating something new. Have you ever thought about writing an unofficial sequel?
New Author, Old Story
Some authors have taken the work of other authors and added to it, successfully. Alice Randall wrote a book called The Wind Done Gone, a novel based upon Gone With the Wind that was not approved through Margaret Mitchell's estate. The Wind Done Gone is a re-telling of the original tale, this time from the point of view of a slave at Tara. There was some legal trouble with the story, of course, but in the end the courts ruled that the novel is a parody -- and that means it's publishable.
Some unofficial sequels have won critical acclaim and managed to stand out for their own merits. The Wide Sargasso Sea, by Jean Rhys, is a modern prequel to the 1847 Bronte classic Jane Eyre. It was even cited by Time as one of the best novels written since 1923.
Would Bronte appreciate the new story? The world will never know -- but it's pretty clear that J. D. Salinger didn't care for the unofficial sequel of his famous book that was first brought to public attention in 2009. 60 Years Later: Coming Through the Rye went through all sorts of legal trouble when the Catcher in the Rye author reacted with fury. Printing of the book was halted, and many legal processes later the book did get published -- in 2011, after Salinger died.
But even official sequels don't always work out. Margaret Mitchell's estate did approve a sequel to Gone With the Wind, a critical flop called Scarlett that wasn't at all popular with fans. However, the book did have all that Wind power behind it. Though poorly-received by almost everyone, it become a bestseller.
Every author draws inspiration from other authors, from stories they've read, from books that have touched them in the past. If you feel compelled to stick with certain characters or stories, why not? Always follow fancy when it tempts you, because you never know just where the next bestseller will come from. But if you're lucky, you'll be compelled to write a sequel or a prequel for a book that's a hundred years old or better. Otherwise, you could be facing serious legal melodrama.