Final Fantasy XV offered a far more open world than previously seen in the series, so I wouldn't be surprised if they take this approach again. In the past we've had world maps that did their best to replicate an open world with the technology limitations of the time, but still these maps did fell open and offered up a lot of secrets, and hidden parts of the world. I always got a special feeling whenever I saw a new town in the distance to explore.
Fallout 4 displays how far today's advancements of open world games have come, and done right and it would be the perfect evolution of the old world map system seen in previous games.
An open world like Fallout 4 in terms of interaction could potentially provide so much more immersion -- the ability to go anywhere, explore every building, and have almost nothing that is just for decoration purposes. This combined with the beautiful art style of Final Fantasy should definitely make for an incredible experience.