The most important thing for me is interesting npc companions (and a lot) just like in BG2 with their own personal quests and banters.
Also interaction between npc's and relationships.
As choosable races I would like to see drow, half drow, duergar and thiefling (possibly these races also for npc's)
I could imagine that the arrival of the illithid could trigger a lot of attention from these races, not to mention the Gith which is obvious.
I also hope that they provide for interesting demon summoning spells.
What also could be interesting is an unkown (not mentioned) daughter of Viconia DeVir and the Baalspawn from BG2. Half drow, half Baalspawn.
Ah, well that's just what sets the Western video game industry apart from its Japanese counterpart. You see, the latter doesn't let popular characters go to waste (they reappear in non-canon games as cameos or playable characters to the point of being accused of making cash grabs or milking their fan base) whereas the former kills them off as soon as they have a few boss scalps under their belt. Just look up the overall RPG sales stats and you'll tell how the Japanese care about their IP and assets more deeply. More importantly, they develop their IPs more successfully.
EDIT: Yes, I know dealing with a character that is made by the player is harder to work with but it shouldn't have been too much trouble importing your BG2 save and adding a few lines, statues or NPCs based on your previous choices. Especially if they were the main character of all the previous installments and DLC! A shopkeep who is descended from the Bhaalspawn and their romantic partner, a temple housing the Bhaalspawn's remains (if your character was a cleric) etc. So much could have been done. It would have added some semblance of continuity to the series. Now series' veterans (or semi-veterans) such as yours truly will be alienated from BG3.
