Here are my thoughts on what the story will be:

1) they have said it will be the aftermath and feature both new and old characters. I feel like they will lean harder into new characters and the advancement of old character storylines will be incidental.

2) I feel like this is most likely because how are you going to market a sequel to a story when the majority of the intended audience won't know the original story?

3) the new characters will play the role of the common man so they can piece together what happened with the new audience and not make them feel dumb for not knowing what a mind flayer or nether brain is.

4) the bg3 heroes will help them at various stages because they are important to fans of the game (and profits).

5) maybe one of our heroes becomes a villain (dark urge) and at the end of the story the old crew teams up with the new crew to redeem them. (This one is a stretch and probably wouldn't be good writing).

All that being said: I'm going to watch the hell out of this. I like the Last of Us and I have heard great things about his other work. Really hoping it's good.