On the weekend, I'm going to be releasing a bunch of threads on common problems - Essay topics will be one of them.
That being said, I wouldn't go near the da Vinci Code.
Historicity of the Christ, the case study, causes enough problems as it is...
That being said, if you're adamant about doing the story, you could look at the film... History as film is a huge source of good major works. You could look at things like how the film has transformed the myth of the book, or something.