Word from Jay Wilson today, indirectly in an odd piece
piece on Kotaku w/o a direct quote on the matter.
So, assuming it's correct, that means just the Barb returns, new and improved, while the other 4 in the initial game launch are new. (Or at least as new as the WD is, i.e. evolved from the Necromancer.)
I can already imagine the massive "which char should return in the first expansion" threads, once we know the 5 chars in the initial game.