Smaug, Ents... That type of stuff is a must. Actually, besides the Balrog, Ents were my 2nd vote... I want Ent figures, but I'd prefer them being all bricks with hinged limbs and stuff so they're a buildable figure basically. Some customization/diversity could be had from that.
Smaug, I just saw someone did like a head of him laying on gold coins and bricks, and it's amazing looking. Again, I could totally see Smaug done from just bricks, and be a kit unto himself. Same with the Balrog.
I believe from what I recall of the early set pics, they are doing an Uruk Hai/Rohan army building set, and I saw a set from the Moria tomb with a Troll, Goblins and the Fellowship...
Don't recall a Gondor set, but who the hell isn't going to jump on a set of Gondorian Rangers? Come on!
One set I recall as well was Weathertop, which looked like a pretty good wraith army builder set of sorts, with a good brick selection.
Oh... And a Sauron Figure from the prologue... Oh yes.
From The Hobbit I'd want elf and human army builder sets for sure... The armies don't show up till the end of course, but there's potential. Some spiders would be dandy too. There's great potential for basically all the books/films here. Lego has a gold mine IMO. There's a pretty loyal fanbase and Legos were just made for this and just never had the shot till now.