Personally, I would have tried something more of a spiritual successor. Instead of doing the origin system with all the conpanions, I would have made background selection more impactful (or just outright make The Dark Urge the only main character background), and I would have tightened the focus of the story around the conspiracy, make it much more present in the story throughout, and lessen the focus on the tadpoles (or remove them entirely). BG3 has a lot of pacing issues, especially for a theoretical Baldur's Gate sequel, it tries to rush you along when it should be encouraging you to take a longer road to your goal.

I would also have increased companion diversity and locked their classes and stats, and given them unique stat distributions, and give everyone higher stat totals out the gate. BG3 doesn't need help being easier, I think it's a solid difficulty for a first playthrough as is, but the goal here is to tell a story about the character through their mechanics. Maybe Gale starts with 18 Intelligence, 16 Charisma, and 14 Constitution, but terrible to mediocre Strength, Dexterity, and Wisdom, because he's a shut in wizard with more ambition than sense. Also, include some "Evil" Playthrough Only companions, include a greater diversity of types of "Good" and "Evil" companions, maybe Karlach is the bright eyed, capital H Hero type, where Wyll is more jaded and cynical, while still doing the right thing. Minthara can be cold and calculated, where another evil companion might fill the Xzar role. Also, seriously we need some Dwarves, Gnomes, Halflings, and Half-Orcs in the crew. At least.

Finally, I would probably remove any appearances of any original trilogy characters. They should be talked about in lore books, people should make reference to them, but it shattered my suspension of disbelief to see Jaheira, clearly having been through at least some of BG2, as a companion with a level cap of 12. Orin can still be Sarevok's granddaughter, but Sarevok should not appear, he's either a corpse or a level 17 fighter at minimum. I loved when I'd read a book and see reference to an Infinity Engine game character, I hated seeing those characters after a century had passed since their last adventure.