I'm pretty sure you'll be able to roam around a fair bit and I suspect most oft he quests have to be manually started, so to some extent there probably will be sandbox elements.

But I don't think the game really sandboxes until you've beaten the main story. Until that point, there's probably going to be things blocking off access here and there. Even after you've completed the story, there's probably going to be consequences depending on what you've decided to do.