From what I can tell and have gleaned from fan speculation and production releases, the characters of Legolas and Galadriel played by Orlando Bloom and Cate Blanchett will be written into the film, as the most blatant examples of artistic license.
Supposedly, because elves are quasi-immortal, and Legolas is described in LOTR as the son of the king of the wood elves, it can be reasonably extrapolated that Legolas was alive and present when Thorin's party was captured by the elves of Mirkwood, and freed by Bilbo Baggins.
Also, because of the quasi-immortality thing, Galadriel can be assumed to have been present and a member of the "White Council", her character can be safely written in, in order to revive another popular LOTR character.
There was an early rumor that Gimli was also going to be written in, and thankfully, it looks like that is not happening.
The other large artistic license will be - as it was in LOTR - a love interest. This time, the character of Bard will be an expanded role, and an elf princess mentioned in The Hobbit (played by "Lost" hottie Evangeline Lily) will have a romantic storyline added as Bard's love interest.
I'll just say it flat out - if the artistic license they take with the Hobbit films is anything like what they did with LOTR, I'll be just fine with it. Jackson's changes were artful and respectful of the original work, and I believe his version and vision of what was necessary to make the leap from page to film was as good as any adaptation of classic literature I've ever seen.
Can't. Wait. For. Beorn.
Can't. Wait. For. Smaug.
