If you want to expand BG3, it would be interesting to add more character strands for good and evil playthroughs etc. or even create new companions for the sides.
It would be a lot of fun to have a goblin like Sazza with you.
There are many generally interesting characters in BG3.
I would like to see an expansion of the depth and breadth of the game.
Breadth means that currently less represented races like dragonborn and orcs are developed more. But also more possibilities to play more evil races like goblins or to have companions like Dro'Ragzlin.
There is also room for improvement in terms of class depth. Let's take the typical Lolth dark elves and the worship of the spider deity. This can also be used to create a class of its own. A mixture of cleric and assa and warrior with a penchant for spider magic, spider conjuration, dark ritual magic, shadow magic and also being able to deal out a bit of damage with weapons.
Again, would be particularly good for an evil path.
An expansion of a druid class that focuses on the bestial path of the werewolf would also be conceivable. A companion that didn't make it into the game has also been leaked.
I could also list plenty of characters that I found interesting in the game or would have liked to have as companions because they appealed to me straight away.
But putting random characters into the game isn't that interesting in the end.
Just continue to expand on what already exists and refine it to make it even cooler.