Ubi have been promising a "true open world" for decades now. It's probably going to have some degree of linearity for the main story, although I guess the multiple choices and routes you can go change things up a bit. Plus there's side objectives and quests you can follow, even if the main storyline is necessary. It'd be nice to see an AC game where you can just forget about the plot Elder Scrolls style and go wandering on your own without a care.