The following is both complete conjecture and a possible spoiler for The Hobbit: There And Back Again, so please think carefully before continuing. But when we spoke to Benedict Cumberbatch on set of Sherlock a little while back, we asked him about his roles in Peter Jackson's new epic, and he said something that may very well be a spoiler, so let's discuss that just a bit.
Cumberbatch said of his role," I’m playing Smaug through motion-capture and voicing the Necromancer, which is a character in the Five Legions War or something which I’m meant to understand. He’s not actually in the original Hobbit. It’s something [Peter Jackson]’s taken from Lord Of The Rings that he wants to put in there."
Now those familiar with the book will notice immediately the big new piece of information here. Assuming that Cumberbatch does mean the Battle of Five Armies (which seems safe), then this points to one way that Peter Jackson and co. have expanded the book's story to fill two films, and provide a more seamless link to The Lord Of The Rings.
Readers will know that the Necromancer is Sauron, and that Gandalf disappears halfway through (the book of) The Hobbit to lead a coalition force and drive the Necromancer out of his Mirkwood stronghold. But in the book they dispatch the Necromancer back to (as it turns out) Mordor well before the Battle of Five Armies. Here, however, it looks like he's going to turn up to the finale in person, presumably at the head of the goblin and Warg army, and face Gandalf's team there.
If that is the case, it's a narratively neat way to combine the two story threads, that of Bilbo and the dwarves and the other following Gandalf and his team. It also gives the goblins a stronger motivation to suddenly turn up: in the book, they're avenging the earlier death of one of their leaders and (like all the other armies present) hoping to get their grubby hands on the dragon's hoard. If they're incited or led by Sauron, however, their actions will hang more coherently with their behaviour later in Lord Of The Rings.
Of course, this is all just speculation for now. We'll know more when we see The Hobbit: An Unexpected Journey this December 14, and see for sure with the release of There And Back Again in 2013.