...how to make a generic character feel as important and meaningful as any of the Origins.
I just don't think it's possible. I think it's a great goal, and it's an understandable want... but I can't imagine how a custom character could compete in a story that could otherwise be dedicated to a known entity.
There are things to do to make the custom character feel a little more invested, sure. NWN2 starts the main character in West Harbor, by the Mere of Dead Men. And this gets referenced throughout the game, giving the main character a sense of home, of history, and of beginning.
The same can be said of a character who begins as a ward of Gorion.
But in these approaches, there's still an empty hole. Something vague and unknown, mostly unaddressed. For instance, if you're an elf, how long have you lived in West Harbor versus a human? Doesn't that have an impact on how long ago the silver sword shattered? It's all vague because it's trying to cater to a thousand thousand players with a thousand thousand different background wants.
As opposed to a story that knows the character. You *are* Astarion. This *is* your history. You *are* Shadowheart. This *is* your history. Now the story can literally revolve around these truths and help you, as the player, get invested. The same way you would get invested in a novel about a character. Now, all of that said, you can still control the character. You can still decide how the character reacts. The only thing you're not in control of is the character's past. That's predetermined for you.
But, in my opinion, that's better. Better to have a concrete history that can act as a roleplaying prompt for you than having a vague, nebulous background that the game is essentially incapable of acknowledging.
And sure, the game can give a generic Tav a special quest in an attempt to provide additional content. That's beneficial and gets closer to making Tav feel important, I suppose, but I still don't see how it competes with an extra quest and content literally tailored to a background.
Just my opinion, of course.