Is there a thread where we can discuss what we think the plot of the movie will be?

This seems to be the thread you are looking for, decarus; over in the GMD forum.

The Magician's Nephew Film Adaptation

It seems increasingly likely that Boy 1 and Boy 2 were actually the same character. From what we’ve observed, there were significantly more girls than boys auditioning. So, if they weren’t finding many standout candidates for Digory, it makes sense that they’d expand the search to an American audience and list the role twice (with slight variations) to attract more interest. 

Some observations about the whole "third child" rumour in the fresh light of today's 1950s themed casting call details

• The new Casting Call states that only two children discover a portal to another world, as per the book 
• A 1950s setting makes a half-indian child in London much more plausible 
• A swimming pool of 40 boys leaves lots of scope for another child to have a bigger role.

I'm inclined though to agree with @impendingdoom's conclusion that Boy 1 and Boy 2 were for the same role, with one just being offered out to try and attract a more diverse demographic, or.to try out some new ideas during the casting development process.