OP, what exactly should be made for it to be a Baldur's Gate 3?
Not the OP, but I think I can field this one:
1. Add Imoen to the game. Only she hasn't aged at all (obviously). And this time we can romance her cuz she's not our sister! BOOM, we call that a Character Arc.
2. Bring back Jon Irenicus. Only now he's a companion. Because remember that time when Sarevok was like, the big evil boss, but then in the next game he was like, in our party for some reason? Yeah, we gotta do that again for sure.
3. Honestly you only need #1 and #2, but if you gotta go for a trifecta, I guess . . . add in more easter eggs and pop culture references that don't fit with the tone of the narrative?