As someone who isn't really troubled by being given a role to play with some preexisting history, I thought the origin chapters in DA:O really made that game, introducing you to the world better than any fetch quests in Candlekeep or long paragraphs of exposition before an intro cinematic. I don't think BG3 would need to make anything near as extensive as Dragon Age's prologues to make something that better establishes the world and your place in it.
Currently the custom MC is like every wakes up with amnesia character, but worse because they actually do have a history that you don't really know anything about. I'm interested to see how we'll learn about the origin characters histories when playing them otherwise roleplaying as them will be all the more troublesome. Astarion NPC, has lived a long life of abuse by his master that has made him self-serving and insensitive to the plight of others, Astarion the PC, how much of that history with Cazador will we learn of in order to play this way, or to believably not play that way?