I would avoid the origin story completely, similar to how Spider-Man was handled in Civil War. Make the main villain Molecule Man who seeks to create expedite the expansion of the human race to other habitual planets, but sees humanity's current form as a hindrance to its overall ability to grow. It would allow the FF to get off of Earth, face a new villain that challenges them on a physical and emotional level, and it brings philosophical questions that can be explored scientifically. There can be nods to Galactus and Doom, but leave them out of the movie completely.