I think the minimum Larian should implement is the Pillars of Eternity Model. I've said this before on this thread, but during character creation, during the "Who are You?" There needs to be like a convo with the tadpole. Asking questions about your backstory. The dream would be origins a la Dragon Age Origins, but that is unlikely to happen, so even just a conversation with the tadpole could really anchor you in the world.

So far, the only thing I've seen that really has these options are Clerics and Drow. Drow have some solid options. But the convo with the tadpole should be things like "how did you get captured?" and for things like if you're a lolth Male drow "What is your opinion on the matriarchal society of the drow?" etc.

I understand entirely creating your own backstory and writing it up yourself...but what if the world later on contradicts the backstory I've written?

Ultimately, more story work needs to be done for the custom character.