Konrad Tomaszkiewicz said that they could do seamless world but they decided not to break the immersion and realism. For him placing skellige islands or kaer morhen next to novigrad or no mands land doesnt make sense.
From: https://www.youtube.com/watch?v=ulXLpibkQXI in polish sadly. (4:20)
I'm wondering how they'll handle it. It could be made pretty seamless if technicalities don't get in the way. You are galloping on your horse past a certain threshold and (without taking control away from the player) it fades to black, "Days later..." it fades in with you doing what you were doing previously. They could probably think of better ways to do it, though.