I'm putting this in the spoilers section rather than the the sequels section, because this all relates to what happens in the second film, so we can't discuss the third film if we don't know the details of the second film.
If you don't want spoilers, please don't read any more.
I know many people seem to think that Galactus will appear in the second film as a cloud, but this quote from Ralph Winter has convinced me, beyond any reasonable doubt, that the cloud isn't Galactus:
http://www.superherohype.com/forums/showpost.php?p=11598103&postcount=33
If the cloud is, as I assume, simply a tool Galactus uses to suck the energy out of planets (sort of taking the place of the big vacuum cleaner he used in the comics), then that makes the prospect of him coming to Earth in the third film pretty exciting.
Ralph Winter did suggest in the Empire article that Galactus could come to Earth in the third film.
Imagine that.
They could bring the Watcher in, or just use the Surfer, but some way or another, the FF could find out:
"Wait a minute . . . you mean the Surfer - the guy who was able to dissasemble and re-assemble people's atoms and could basically destroy any one of us with the blink of an eye - and that cloud that nearly destroyed earth are really nothing more than a spoon for some guy called Galactus? And now that Galacutus' spoon defied him, he's coming here? And he's pissed?!?!"
t:
That makes a pretty exciting possibility for a third film. My only concern is that they could tell the story of Galactus' punishment of the Surfer in a stand-alone Surfer film, but my hope is they would bring Galacutus in for the third FF film, and then spin off a Surfer movie prequel in which they tell his origin.
Two questions:
1. How cool would that be?
2. How likely is it?