I am hoping for 3 things, one of them a "game breaker", I suspect:
1) BG3 remains more of a "solo" version of the RPG line, with a new character. BG2 had a real end. There are already plenty of MMORPG-ish games available, and game design, balance and plot creation for the 2 are worlds apart.
2) The world be alive. This is a real nightmare for the game designers, because there is a tremendous amount of creative effort required to do this with the NPCs, and game designers are not necessarily talented storytellers. This also means using a system that allows the world to react to you, as your alignment is formed/changes/whatever.
3) In addition, for the game to be alive, the plot needs to be able to handle real player choices. This is the insanely impossible task for game designers with a static product. To be honest, real role-playing character paths for differently-aligned characters should be wildly different. They did do a fairly good job of this in BG2, by giving you two motivational options to do close to the same tasks, but even there there were tons of anomalies because the plot was really designed for the good player. I should say, "I suspect", because I refuse to be an active, evil agent in an evolving story (to each his own - I am just showing that the anomalies are based on estimation, not practical experience).
So - I will be happy with 1 and 2 above. If I really want a dynamic world, I will have to find some GM somewhere. I think, though, that I will stick to my original decision 20-some years ago, and live with the dynamic evolving story of *my* life, instead - leaving RPGs to have a beginning, a middle, and an end
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