I think a DA:O style background system would be a bit too complicated for the amount of races and classes that will be added to the game.
That said, I do really like DA:O's system even if I desired a bit more such as Apostate Mage and Daelish Elf Mage as options.
I don't think we need anything as elaborate as DA:O's system, but this might be the right direction to take.
1) Create entirely unique background tags for custom characters based on their place of origin. We already have the Baldurian tag. Add in a couple other locales, the most obvious ones being Waterdhavian and possibly Chult.
(Chult is a tropical island nation that has already been indirectly referenced within BG3, as there is a note at the harpy nest that seemingly references the Tomb of Annihilation module and its villain. This note appears to be completely original writing, as in, it's something Larian made and placed within the game themselves. As someone who did participate in an ultimately failed (as in, we wiped) tabletop campaign featuring that module, I am intrigued to see how this may later factor into BG3's story, and I can already think of a few reasons why this could be potentially important later. Especially in regards to the big bad's goal in that campaign.)
The big bad of Tomb of Annihilation was a lich named Acererak. To my understanding, and long story short, he was responsible for a curse that drained the souls of everyone that died, which made resurrection difficult and even temporary in some cases when it was performed. He was funneling the souls towards some kind of being that he had transported from the Astral Plane in an effort to transform it into a god, and presumably setting it loose within the material plane to ravage it once the process was complete. The goal of the campaign once the cause of the curse is discovered is to kill this being. Acererak does engage the party in combat afterwards, but the party can only try to survive and repel him. Interestingly enough, there is a clear distinction in that he did not intend to give himself that godlike power.
2) Those background tags should result in custom characters having meaningful interactions with the world that the Origin characters simply cannot have, the same way that custom characters cannot have interactions that the Origin characters do. This was the major thing that DOS2's customs lacked. For example, Gale talks about Waterdeep and the Yawning Portal after the scene with the goblin prisoner at the druid grove. We are unlikely to have another Waterdhavian companion, so a custom MC from Waterdeep will have access to an interaction not available to any of the origin characters here.
Granted, in regards to the rest of the world, Gale's Waterdhavian tag would probably act the same as a custom MC possessing it too, but there should be divergence here. Gale was a wizard that may have largely stayed above ground within the city. A custom MC should have the option to define if they were someone that also largely remained within the city, or if they were an adventurer that would frequently go through the aforementioned Yawning Portal to explore the Undermountain. Very different things. A custom MC that did the latter should definitely know more about Drow, and it may also explain how they got grabbed by the mindflayers to begin with. A custom MC that did the former would likely be a lot more aware of the political climate of the city, and I can imagine them making snarky remarks to Astarion about how Baldur's Gate and its nobles aren't anywhere near as grand as Waterdeep.
Though given that Gale talks about the Yawning Portal and his connection to the Weave, I wouldn't be surprised if he actually did venture into the Undermountain and met a certain important mage there at some point too.
