For me, that would be doing a crappy Galactus story. Humans don't fight Galactus. Or shouldn't. If Galactus is fightable, that kind of defeats the point of the character.
When the FF first meet Galactus in the comics, while the others keep Big G distracted, the Watcher Uatu tells Johnny Storm how to sneak onto Galactus' ship to steal a gizmo, the Ultimate Nullifier, to drive off Galactus. There is no real fight, even with Silver Surfer present. The heroes don't win by strength or guile, nor even perseverance. They win because a powerful alien force likes humans, and handed them a solution. It worked fine in the comics of the 1960s, but in a modern big-budget action flick, having the heroes be pretty much utterly useless against the antagonist is probably not the way to go.
And, as I said, I don't think the FF currently have the cachet to do a two-movie story.