A giant cloud.
According to Moriarity (probably my favorite contributor to AICN) someone sent in a scoop regarding what the big-G will look like in the upcoming film. I think I felt a collective scream in the Fanboy collective upon reading the news.
Here's a quote from Tom Rothman, stated back before the first Fantastic Four movie was made:
"There won’t be any giant robots in any movie my studio makes."
This, of course crushed the idea of ever having Sentinels appear in any Rothman-helmed Marvel superhero flick. (Sentinels being the 10 story tall robots that were created to hunt down mutants.) And since Galactus is portrayed as a giant the size of a planet in the comic books, I guess he fits under that category.
In September of last year Rothman had this to say in regards to Galactus appearing in the movie:
"Yes, I think that Galactus, look, I don’t want to give away too much, right, but I will say this, Galactus will appear and fans will not be disappointed."
Now remember that five months later, in an interview Ioan Gruffudd stated the following regarding Galactus' appearance in the film:
"...his presence is felt in the movie. He doesn't physically appear, no."
Now I'm not saying that a giant dude would necessarily work as Galactus in the movie... sometimes things need to be changed from the comic book version in order to fit better into a live action movie. However, having said that, here's an image a fan did on his home PC using just Photoshop:
So yeah, I think it could have worked.