Therefore I think that we will actually have two completely different routes, not least because Larian so often hinted that 'you can have two completely different adventures'.
Maybe if you only follow the critical path. Datamined content confirms we'll be moving between the Underdark and the Shadowlands for the Nightsong quest at minimum. To say nothing of the githyanki creche, another suggested route that appears to bypass the Moonrise altogether, which I doubt will be allowed.
Most choices will probably be Morton's forks. Developers have always been wary of locking players out of content based on their choices. The only major game I can think of that went through with the promise of completely different story paths was Witcher 2, and it was criticized by the players because they didn't want to replay it to see the other path. From the development standpoint too, it's not an efficient use of resources to create content only a portion of the players will see in one playthrough.
