Yeah I just don't see this working without McKellen, Serkis, and Holms.
I never read The Hobbit so I'm not sure if any of the other characters from the LOTR trilogy made appearances. My only exposure to it is of course the animated "movie". As awful as that maybe I really enjoyed it all 50 times I've sen it.
Its been forever since my last viewing but I'm betting John Rhys-Davies would make a great voice of Smaug the Dragon.