Brook
We don't know anything about his childhood years and early adult years except that he used to be the guard of a certain kingdom. Plenty of stuff to cover. The most mysterious
Franky
Abandonned by his pirate parents which we know nothing about
Zoro
We know nothing about his actual family and how he ended in Shimotsuki village
Nami
War orphan as a baby, got some kind of unexplained supernatural unique ability to sense the weather highlighted by Vivi (special bloodline?).
Oda does not have to develop them, maybe Brook's life before he joined the rumbar pirates was simply uninteresting, Franky's parents were random fodder pirates, Zoro's family were peasants, Nami's weather sensing ability is simply a gift she was born with and her origin is irrelevant but I think those 4 are the ones Oda could add a new background if he wants to.