Sounds like a Bethesda game. Hopefully they can do it without copying all the npcs, and maybe make the side quests interesting. That's a lot of content to keep at a high quality.
CDPR did an interview with GI talking about the open world. They specifically mentioned this being the focus of their concern. While they loved Skyrim in the end the Bethesda chose to concentrate more on the world than the characters in it and this is where they want to differenciate themselves. They are aiming to keep their strong and multilayered characters while allowing the freedoms of open world gameplay. So at least they are going in trying to tackle this problem.
Anyway heres some other information I'm getting from some of the other interviews.
-At least 3 regions. Skellige Islands (viking like), Novigrad (cloak and dagger like), and No Man's Land (black magic is mentioned).
-3 different epilogues depending on player decisions, each roughly an hour or 2 in length.
-Monster Investigation is Important. If you do your research, you'll gain tactical advantages in combat. For example if your fighting a vampire and you know it has a poison gland because from reading a book, you can spend Witcher focus to strike the gland and take away one the creature's special attacks. If you go in recklessly the fight will be tougher.
They are currently working out how they want the system to function. One idea is a VATS like system (personally I think they mean more like the adrenline system of the W2). The other is a special move you do in the flow of combat.
-Imports. PC this should be easy, but they are unsure how it would or if it will work on consoles.
-Mentioned about wanting to improve inventory again.
-Working hard on no loading screens. Also they wanted to get rid of the slow moving through doorways loading, they had in the 2nd game.
-Approx 50 hours for the main quest. 100+ including side quests.
-You still need to drink potions before battle BUT you can choose when to activate the potion's effects. Witcher's control their metabolism so you can decide when you start processing certain agents.
-From the Russian site Igromania: The combat is somwhere in between the one of The Witcher 2 and the one of The Witcher 1. It is slower than in The Witcher 2, and requires more thought. The number of different strikes and combos increased fivefold, and they heavily depend on timing. If you press the button a second earlier, Geralt will strike in one way. A second later, in a different way. And the developers said that, citing them, "Geralt will no longer turn his back on foes".
-They want to make the combat less of a slasher. The roll is out because they felt it was abused. Pirouttes are in.