I'm sure it's safe to assume that Blizzard has thought this out quite a bit. I'm sure they knew that many fans would be more than happy to see another race added but ONLY if it could be worked into the game to be balanced as perfectly as the origional three were. I'm guessing they gave it a try early in development but just wasn't meshing right maybe? I feel that if they were going to surprise us with a new race at some point they just would have avoided answering an questions pertaining to it and then out of nowhere announce the new race.

That's just my view on it. Though I guess it's always worth hoping.