Things that have been revealed
Thrain will be involved in some way — he and Gandalf will meet in Dol Guldur, and his presence will help tie together the dwarf and Necromancer stories
The Nazgul will be present (a stuntman involved in the film stated that he’d been performing “ringwraiths”, which would suggest that there will be more than the single Witch-king cameo we saw in An Unexpected Journey. And as we know from the released scene in the High Fells, all nine of the Nazgul tombs are empty.
Galadriel’s promise to Gandalf, that if he needs her she will assist him, will probably be upheld — potentially with the results for Dol Guldur being Galadriel “throwing down its walls and laying bare its pits” as in the Legendarium in Appendix A of The Lord of the Rings. Though, from the general appearance of the fortress, one wonders if she may not have done so already.
We’ve also heard that Gandalf will be captured (or even incapacitated), and at one point will be rescued by Radagast and/or Galadriel.
All of these seem as though they could play into the Battle of Dol Guldur, or events immediately pre-preceding it.
However, there may be much more the Dol Guldur story than what has been established — elements will be revealed as the narrative expands in The Desolation of Smaug.
Could Sauron be the “big bad” of the Desolation of Smaug?
In my opinion, from what little we know — Peter Jackson may ultimately cast the Necromancer/Sauron as the “big bad” again in The Hobbit, with Smaug and Azog as a potential ally and a captain, respectively. Orcs and dragons owe their existence and their allegiance to the Dark Lords of Middle-earth — both Sauron and Sauron’s master, Morgoth.
Could The Desolation of Smaug reveal that Sauron is moving to assail the Free Peoples from Dol Guldur, with the aid of Azog and the potential assistance of Smaug? The recent movie tie-in book Desolation of Smaug Annual revealed that Azog had destroyed the Beornings, and that he is attempting to prevent Thorin from reaching the Lonely Mountain.
Is Sauron beginning to destabilize the North-east around his domain, and trying to ensure that Smaug remain exactly where he is so that the Dark Lord can obtain the allegiance of the worm? And then, there are the rumors that Sauron may return and even raise Smaug for The Battle of the Five Armies in There and Back Again.
Benedict Cumberbatch has mentioned that he has a part to play in “The Five Legions War”. But these are remain simply rumors and speculation for now.
Just how close or far away from the mark these theories are depend on:
A. How closely Azog is associated with the Necromaner is The Desolation of Smaug
B. How the new players in The Desolation of Smaug (Beorn, the Wood-elves, the Lake-men) fit into the Necromancer story — if at all.
