About origin characters being added in Act 2 onwards, as it would be necessary to have more options in a game with 6 party members:
My thoughts about this is that it's not mandatory to have a very complex and deep origin story for all future companions.
For most NPCs developing their personalities, range of emotions (not the 50 shades of arrogance like DOS 2 have), backgrounds and making them appeal to players while adding one/two specific mission is enough.
All NPCs should have specific roles, and being considered a great addition to the party, while being very different from each other, and possibly having their own unique thing (being a vampire, like Astarion).
Working into personality is important, so the player doesn't have this artificial feeling that the NPC was there just for gameplay reasons.