Concept: a fantasy setting where True Names are extremely important in magic, but what makes a True Name is wildly inconsistent from person to person. Some people’s change; some people’s don’t.
Maybe it’s the name they were given when they were born; maybe it’s a name they chose for themself. Maybe it’s whatever they are most commonly addressed as, which makes keeping it secret rather complicated. This person’s true name is from their adoptive family, that one from their birth family, that one from the family they married into. That person’s true name is the nickname they were given by their dearest friend. That person’s true name is the nickname they WILL be given by their dearest friend, whom they haven’t actually met yet. That person’s true name is the nickname they were given by their dearest friend when they were in preschool, who they haven’t seen since. That person’s is the name of their self-insert character.
Unless there’s some way of divining true names, there would be a lot of people who just don’t know what theirs IS. (If there is, many of them would still be left wondering where the hell their name comes from.) Very powerful name magic becomes very TRICKY.