The Custom character will have unique elements compared to the origin characters because the custom character can choose a race, background, and class and subclass that the origin characters can't, so if no origin characters are say a Tabaxi (if its in by launch) Beastmaster Ranger with Acolyte background you will have a very unique experience from the Origin characters. Plus the custom origin mention free destiny so that might be a unique plot point for custom origin characters. Plus maybe they will make them better at using the tadpole then none custome characters.
