I don't think that a backstory is required to give our Tav a story arc. They just need something more unique to hook them into the story that's different from the other characters. People get caught up on the idea that backstory is a requirement but it does not have to be. Like, what if the origins were all more limited in what they could do and what endings they could achieve, and only Tav could get access to all the endings we have now? Or what if Tav started out the story meeting a particular character unique to a Tav playthrough, some recurring character that we can have a unique relationship, whether positive or negative, with? Hell, maybe instead of the nonsensicle nautiloid kidnapping that opens the game, we can get kidnapped by a particular character who could be a recurring enemy for us, someone we as Tav could develop an arc based around our feelings toward. Just something to anchor us to the story. I'm currently replaying Pathfinder: Kingmaker. That game starts with us as just a random adventurer who answered a call put out for random adventurers. But implicitly, we're the ones always driving the story forward. We're there because we chose to be there, not because we're forced. We take responsibility for creating a barony, something our character chose. In BG3, we can choose a lot of little stuff, but our Tav doesn't actually get to drive the story ourselves. We're not the centre of the story, the story is just happening around us and to us, and we're just there to experience it.
And I feel the need to mention again that the opening sequence with the nautiloid really does get dumber the more you know and the more you think about it.
