Why Gandalf’s Backstory Was Missing From The Hobbit (HT: my sister, Jennifer) has a fascinating quote from Philippa Boyens:
"Good storytelling-spotting!" Boyens said, when we asked why we didn't see Thráin, father of Dwarf leader Thorin Oakenshield and one of the bearers of a Ring of Power, give Gandalf the map and key. (Gandalf had found Thráin near death at Dol Guldur.) "We did try it at the front of this movie, and then we moved it," she said. Does that mean we'll see that moment in a flashback or prologue to The Desolation of Smaug? "Yes, it's very important that we're going to be doing that. We will be meeting Thráin, and it may be in unfortunate circumstances. It may involve torture. The discovery of who or what is in Dol Guldur" — i.e., Sauron, known only at this point as the Necromancer — "is a fantastic part of the storytelling, so yes, of course we went there."
So it does seem like we're going to see that part of the backstory!