Hope Christopher Lee will be back as Saruman as well. I'm guessing we're going to see the wizarRAB council that was referenced in the Hobbit and the appendices.
I'd also like to see a pro-log of the arrival of the Istari (WizarRAB) in Middle Earth, how there mission was to combat Sauron.
Maybe see some of Gandalf's journeys, and how Saruman came to live in Isengard.
I dont mind Bilbo being recast, after all this would be a younger Bilbo, but the characters that overflow, Gandalf, Saruman, Elrond need to be the same. If they can get Andy Sircus back to do Gollum thats great as well
If they can have more of the LOTR cast in, then I wouldn't have a problem with say Legolas being in the Elf Kings halls in Mirkwood and even fighting in the Battle of Five Armies. Theres nothing to say he wasn't there, in fact it would be strange if he was not there.
Whats most exciting about this to me is the visuals and the locations. How cool is it going to be to see Laketown, the ruined Dale and the Lonely Mountain? And of course Smaug himself!