I'm not saying Bluepoint will introduce a new area to be accessed by this archstone, but I'll say this:
If I were adding a new bonus area to Demon's Souls, I'd make sure the sixth archstone was broken at the start of the game in order not to give that fact away.
Didn't they already confirm shortly after the first reveal that there will be more content than in the original. Like more weapons/armours/enemy types and areas?