If it could be done in a fitting manner, The Night Circus would be absolutely stunning. It would need to be a film and you'd have to have someone artistic like Guillermo del Toro - though not necessarily him - to direct it. When I read books I don't imagine them with a specific cast in mind; the freedom to imagine the landscape and the characters as information comes across little by little is the best possible thing about a book for me.
One must would be though that Celia be played by an American and Marco someone English. Bad accents or having to try harder than necessary to envision a character rather throws me off.
I'm sort of imagining a cutout (stop-motion) animation title that bleeds into the opening scene. I could go on, but, you get the idea.