I know Galactus is overused as a threat, but I like the kind of conversation Hickman wrote between him and Reed last time, and I could definitely go for more of that. I'm pretty sure the teaser is just re: the Council of Reeds attempting to destroy the world to make their gateway back home in the current F4 storyline. Galactus isn't threatening the Earth, he's just chillin' on the moon, hoping for the chance to pick up a free snack if somebody else destroys it for him.

Plus he's already said he doesn't want to get near Earth himself because he's scared of Franklin.