"Total sandbox" is impossible, and "totally linear" is not on the table, but Larian in an effort to marry these two, gives you the option of play pre-made characters with bespoke storylines and with established histories and personalities, or creating your own custom main character who can be a blank slate for the player to write on.
I think this is a laudable endeavor for Larian to attempt but I'm afraid that in trying to be a jack-of-all-trades it won't be able to give us enough of either type of play-style to really make a good story. I feel this way because that's how I felt about DOS:II, I didn't think playing a custom character was worth it storywise, and I didn't think that any individual origin storyline satisfied me from an RPG, 'choose your adventure' stand point.
I find total sandbox games like a lot of the Rogue-like games that have been coming out terribly dull, I come this genre for the interactive story telling, "emergent gameplay" doesn't really interest me as much, so I'm inclined to want a narrative with a more established character but ideally I would like a game that let's be create a custom character that would be able to interact with the world with the same kind of depth as an origin character.
Funny to read that, as some of your points are very similar to a part of a long post I'm (very slowly) writing.
Larian is aiming at something I consider a step forward for RPGs (and games in general) - both custom and fixed protagonists to choose from - but it's hard to get right and not getting it right is going to result in both options being underwhelming at best. It could be truly amazing if both were done very well, but that is both difficult and requires lots of resources.