There are actually only few thing which I'd love to see implemented in the game:
-A housing or a stronghold system which would allow us to have our own base of operation. It can be similar to Lady Vengeance but I'd like to see more interactions between the people inside of it,
-Interesting romances with companions. I actually like the romances in Divinity Original Sin 2 but I'd love to see the romances being much longer with plot twists. Maybe something similar to Dragon Age Origins? This game had good relationships if I remember correctly,
-Many dialogue options and choices which influence the world around us. I think that this is one the most important aspects of the game which has to be taken care of. Can you imagine an RPG without good conversations? I doubt it,
-Alive world and by saying that I mean seeing the people in the town react to the environment, react to the player's class, race and react to our companions.

I think these are the things which I would like to see in Baldur's Gate 3.