There's a time honoured tradition of borrowing characters from someone else's fiction. Even Shakespeare did it! Just take a look at this list.
An article in The Age tells how Sherlock Holmes and Mr. March, the father in Louisa M. Alcott's Little Women are among the latest characters to be given a new lease of life by contemporary writers. Peter Carey is featured in the article (this being an Australian newspaper after all!). He, of course, based his eponymous ('scuse me, but I love that word!) Jack Maggs on Magwitch from Dickens' Great Expectations, and My Life as a Fake is in parts a retelling of the Frankenstein legend.
One of my favourite books of all time is a sequel of sorts and far excels the original in my opinion. The Wide Sargasso Sea by Jean Rhys is an amazingly sensual and powerful novel which borrows characters from Charlotte Bronte's Jane Eyre. Mr. Rochester the romantic hero of Bronte's novel is very much the villain of the piece, driving his wife beautiful Creole wife, Antoinette, to madness.
Which fictional character would YOU like to see in a sequel or prequel? I'd love to fill in Captain Corelli's missing years ...