Films have to "take liberties" with books - a straight transfer would be boring because it's a different medium. Most of the dwarves in The Hobbit have almost no characterisation, so expect big changes there.
Legolas is Thranduil's son, so no reason why he cannot appear as one of the Mirkwood elves.
Mr Rhys Davies may return, though he did vow never to wear prosthetics again.
Gandalf makes reference to an order in The Hobbit that implicitly involves Saruman (and Galadriel and Elrond, I think), so we may see Saruman again - perhaps before he was corrupted.