They say that the what you start with and what you end with will define everything about the story--once you have these two figured out, the rest will fill itself in. So, assuming the general plot stays the same throughout, we need to figure out how to make all that make sense.
First, this is a sequel, for all intents and purposes. Therefore, a "Previously" is always helpful, and that will be our jumping off point.
The film would open up with a child staring out his window at the moon, his nurse moving quietly in the background. The boy cannot sleep; and so the nurse offers to tell him a story. She pulls a book out, opens it, and begins to read. The camera tilts down to look at a picture on the book: four adults, two female and two male, riding horses. Transition to actual footage of the Pevensies. Nurse's voice over would continue, giving a brief chronicle of the Pevensies' reign (shots of Lucy healing soldiers on a battlefield while Peter guards the area, Peter and Edmund fighting back to back, Lucy and Susan hosting royals from Archenland, Edmund and Susan working with matters of state, Lucy and Edmund working with the Narnians, finally the hunt for the white stag). The nurse's tale ends, and the boy asks what happened to them. The nurse closes the book and says only, "I don't know, Caspian... I don't know."
Transition: Nurse stands, her clothes moving into the camera's view and morphing into the blue of Lucy's jacket.
The Pevensies are on their way to the train station. Everything is fine until Lucy and Peter get separated from Edmund and Susan because of the crowd. During the brief confusion, a boy shoves Lucy to the ground to get her out of his way. Peter rushes to help Lucy up, and sees a fresh cut across her lip. Peter turns away from Lucy dives on top of the aggressor. This is how the fight begins, but in this version Peter is winning, pummeling the boy furiously. This shows that Peter is still fiercely protective of his younger siblings, but no longer goes about it in the right way. Lucy pleads with Peter to stop to no avail, until Susan and Edmund catch up and Edmund manages to talk Peter down, while Susan tends to Lucy (and ignores Peter, clearly furious about the fight). This shows that Edmund is still the Just, and learned his lesson from the last film, and that Susan is still Gentle--but is starting to fall away from her siblings.
The four continue on their way, and the film progresses as normal. This beginning would help clarify some of Peter's antagonism towards Caspian--when they first meet, he believes that this man was a threat to his precious little sister. In addition, it shows that Peter has begun to stop listening to Lucy, it shows how Caspian found out about the four rulers, it makes Susan's warrior stint all the more shocking, and gives a preview of why Peter might choose Edmund to deliver his challenge: he's good with words.
So... yeah. That's a rough idea of how it could've started.
). I thought that was a bit cheesy, since a good and thorough assassin would have checked the wardrobe, once they realised there was only an extra pillow in the bed.
