Why Not Demon's Souls 2
Miyazaki said that it's difficult for him to give a straight answer to the question of why the new game is Dark Souls and not Demon's Souls 2. He did want to make Demon's Souls 2, but "various things" prevented that from happening. Regardless of this, he's happy that the game has become Dark Souls, as it allows him to show his creative side by using new ideas and not being tied down.

Fully Seamless World
Asked about new concepts for Dark Souls, Miyazaki said the game places focus on the feeling of exploration by making the map fully seamless. 4Gamer asked if this means means the fields will be like the open fields of Bethesda's Oblivion series. Rather than this, he suggested that we think of it as being a full dungeon. Demon's Souls was split into stages like 1-1, 1-2, 2-1 and so-forth. Dark Souls has these seamlessly intertwined.

Walking about the fully connected map is very enjoyable, said Miyakai. The map portion of the game is still not complete though. It's like being between alpha and beta state at present.
The map isn't just expansive, Miyazaki noted. They've prepared a variety of situations. There's more than just Dark Fantasy. You'll also find High Fantasy areas, hell, and other varying maps.
The world will also have great height, although this is something that was true of Demon's Souls as well, Miyazaki feels.
In making the world map, they're considering things like how players will shift viewpoints, general movement patterns and so-forth in order to create a map that's easy for players to get a grasp of. They're keeping in mind how players remember the map in their head and are taking care to not exceed that processing limit.

The rest of the interview can be viewed here - sos