Demon Hunters will "go forward" like DK and Monk. You'll be able to make one and play one. Nothing else will change.
BLIZ has said they would CONSIDER that maybe in the future, more races are able to train as Demon Hunters because it is very limiting but that they wanted, atleast, the launch to be about the races that have always been associated with them.
Personally, I'd rather they not until they fix the game file conflicts the D.H.s created.
That was not welcome at all. So if they never expand? Fine by me. We don't need more issues.