I honestly think it is too late to implement anything like that. For this to be actually good (and it would need to be; I can't imagine the outrage of the community if this feature was half-assed) they would need to make this 'custom quest and backstory' very vast and impactful. Otherwise it wouldn't feel good enough, afterall we all have high expectations (rightly so, I believe).
Something that I think might be possible is to have a choice were you come from roughly, to introduce some dialouge choices (like the 'Baldurian' tag you can get already). I don't like that you are tagged with 'Baldurian' as soon as you play a half-elf, for example, so having a choice here would be nice.
A few rough choices of origin could go a long way already, and might be the only feasable option this far into development. At some point they would probably like to finish this game.
