From the Reddit AMA:
1) Will Baldur's Gate 3 be moddable? If so, is it possible for modders to add in new locations/companions in the main storyline or add in new UI?
We're focusing first on developing and finishing the main campaign, once we have some space, we can talk about that.
Will there be a Dungeon Master mode in Baldur's Gate 3, similar to the one you had for Divinity Original Sin 2? I really enjoyed it and with the Dnd rules it would have even more potential.
When we built GM mode for Original Sin, we were of course thinking of D&D and how long it had been since anyone had made such a mode. So yeah it'd make a lot of sense, but we're focusing on developing the game first at the moment...
Not very encouraging, at least for release. I really hope they don't drop the ball on this.