-More BG2 characters returning, including Edwin the Lich, Jon Irenicus (specifically whatever monstrosity remains of him), Viconia (original VA), Jan Jansen, etc.
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Somewhat joking, but the Dead Three decide to put some of their Divine Essence into the tadpoles as a means of weaponizing the Cult of the Absolute to its greatest capacity, but something goes wrong and we end up with the consciousness of the Bhaalspawn. As a result, somehow Sarevok returned.
I’d be totally up for more BG1/2 character surprise appearances, if they can be convincingly tied into the story, and didn’t require knowledge of the previous games to make sense. Eg Edwin turning out to be linked to the Lorroakan plot lines, or Viconia being behind Shadowheart’s Sharran mission to steal the box. Jan Jansen … possibly not so much

. His rambling stories were bad enough when they were mainly text, I’d be screaming if they were voiced. Still, better him than Aerie!
Regarding the Dead Three, I would love to be surprised by Bhaal being tied to the main story in some way that helps tighten the connection with the Bhaalspawn saga and make the three game arc hold together as the story of, say, Bhaal’s return to godhood. I don’t think this is necessary in order to make BG3 a sequel to the old games, though, and am not sure if or how it could be done well, and without compromising accessibility of the story for players who haven’t played the original games. Perhaps any shenanigans involving the portfolios of death gods will remain a side plot.
I do hope that Larian will continue to fudge the story of Gorion’s Ward as far as is consistent with having a strong story and fun callbacks to BG1&2 in this sequel, though. It would be a most unpleasant surprise if Abdel Adrian were referred to in the game, or if much more from the originals was made canon in BG3 beyond what is essential to have the characters we meet in it alive and, I guess, Gorion’s Ward having finally died around 10 years before BG3 starts to enable Bhaal to return.