I really hope they will use the open world as a big part of the game design, so features like this would definitely be a good idea. But it's kinda hard to imagine to be honest what they might come up with because, as far as i know, an open world APRG has not been made yet, at least at this scale. I'd like to see them take ideas from other great open world games like The Witcher 3 for example; one of my favorite things about that game is all the side quests and sub plots you get to experience from exploring the world. I'd really love to be able to explore Sanctuary in that way.
But since Diablo is an ARPG they have to handle it a bit differently - it has to be a much faster paced game and still work well with exploration - so they need to find a good way to balance it. But I feel like that could be a hard thing to accomplish if the game will be as fast as D3 was; i'd like it to be much closer to the pace that D2 had, not just because it fits better with an open world, but it also feels almost impossible to design strategic and thoughtful combat (which they said was the intent) when it's too fast. In my opinion, to achieve this, stepping back a little bit from the action and focusing more on the RPG aspect would be a smart choice. Most importantly though, i just hope they don't make an open world just for the sake of it and because it is impressive, but actually design the whole game around it. At least they said that they are "building Sanctuary as a living, breathing character" - which does sound pretty promising