I expect Smaug's original attack on Dale and the Lonely Mountain, the doings of the White Council and Gandalf's trip to Dol Guldur (in the category of flashbacks) will anchor much of this, but think of all the dialogue and sub-plots that don't really exist which he'll have to invent. Ugh.
And what's next? Characters that don't even really have basis in the books? As annoying as the Arwen/Glorfindel merger was (for example), at least Arwen's early lines in Fellowship all had foundation in something Glorfindel said or did.