Yeah, I hope it's integrated with the main game maps and not be separate. Shadow of Mordor did the same mistake, the expansions were great with great new additions but making them standalone really pissed people off. Was a shame really, hope CDP doesn't go that standalone route.
Making the larger expansion separate from the main game would have its advantages, though. It would avoid problems related to balancing the gameplay, as XP/level and gear from the main game would not affect the expansion (and vice versa). It would make it possible for characters from the main game to also appear in Toussaint. Bugs related to integrating the expansion and its quests into the game would be avoided. It is not without a reason that other games choose this approach.