'Fictional' and 'fanciful' have quite different meanings. 'Fictional' simply means something was made-up—it has a neutral connotation. 'Fanciful' is a less common word that means 'over-imaginative and unrealistic' (OED); it has a negative connotation. For example, if I say something was a 'fanciful proposal', then I am suggesting that it is unfeasible, and perhaps that someone was being over-optimistic or naive when they made it.
Edit: 'fanciful' has another meaning, which probably applies to your question. It can mean an 'imaginative take on something' and does not necessarily have a negative connotation. In this case, 'fictional' and 'fanciful' are roughly synonymous.