Most of the appendices is stuff that's unusable (stuff on calendars and languages), stuff that's not relevant (the detailed timeline of the 2nd and 3rd Ages) and stuff that's already been used in the trilogy (Arwen and Aragorn's romance).

What's left is basically 'Durin's Folk', which contains a couple of slivers of additional info on Gandalf finding Thrain and getting the idea of taking down Smaug, and that's about it. What they really need is 'The Quest of Erebor', in Part III of UNFINISHED TALES, which contains highly detailed information on Gandalf's descent into Dol Guldur, finding Thrain, meeting Thorin, proposing the attack etc. But they don't have the rights to UNFINISHED TALES and Christopher Tolkien has cheerfully said they'd be sued into oblivion if they tried to use any of that material without his permission, so that's not possible.

So my guess is that a lot of this material is going to be pulled out of someone's backside. I'm just hoping that THE HOBBIT will still be the two films and this third movie will be 'bridging' material to the LotR trilogy, which can be safely ignored if its garbage.