It's bad from a Fair Use perspective if your work would negatively impact the financial gains of the copyright owner. This is more true when the impact is obvious and foreseeable, but courts still allow some speculation here. For instance, even if an author has no intention to develop his book into a screenplay, the court recognizes the economic value of his right to later choose to adapt a screenplay if he wants A large consideration for the court is whether your work could be said to "fulfill demand for the original". If it can, your Use may not be Fair. When your impact is more speculative or uncertain, the other three factors matter even more.
Click here to learn if you are protected by Fair Use