Whilst the trilogy is mostly based on the one book, they're adding bits in and possibly partly closing the gap between the end of the Hobbit and the start of LOTR, based on the back story of the LOTR characters, and the Unfinished Tales. If any extras follow reasonably close to Tolkien's writing it might include more Elves, and also Strider ( Aragorn ), who captures Gollum in the Dead Marshes.
( Mind you, Jackson did make some notable 'variations' to the established story line in the original films ).