My dream version: more halsin and mithara content.
Mithara doesn't recruit him as a good person.
More data, interactions and possibility of romance between DU and gortash (a previous relationship is hinted at several times but everything is unfinished).
Gortash doesn't have to die.
Possibility of romance between DU and his great-niece, it's a family tradition, I didn't invent it.
Why it is mandatory emperor or gith I don't understand, you should have the possibility to choose one, the other or pass a roll for them to join forces.
Please, justice for Sarevok and Viconia, if you're not going to respect them change the name because they are characters you have invented (note that I was the happiest person to have the characters from previous BGs back until seeing these).
Misc and janeira are mandatory for me.
Is Upper City too much to ask for? It's a dream BG3 so I ask for it.
Consequences to get up to your neck in eating worms. Decent evil endings.
Corrections of inconsistencies in the story.
A little more relevance and politics in Act 3 wouldn't be a bad thing, since the issue of the three dead being behind everything at the end is almost irrelevant.
More camping scenes.
More options for interaction with your romance (it's not too hard to add a "let's go to bed" like in DO).
Will is a pain in the ass to me and I've read several times that if the original version had more substance and personality, I would like to see a Will like that.
But above all, correct plot holes, various inconsistencies, adjust things to the real D&D Canon...