What is Madeleine's big secret?
Right at the start, she says she has a secret she will share with Bond after he has visited Vesper's grave. And Blofeld reinforces this, telling Bond that Madeleine has a massive secret that is going to blow everything apart, when it gets out (or words to that effect).
But what is it? It can't be Matilde because she doesn't exist in the 1st scene and that wouldn't fit with what Blofeld says. And it can't be Madeline's association with Spectre, as Bond knows all about that. Bond doesn't necessarily know about her childhood trauma, but that wouldn't fit with Blofeld's prediction either - Blofeld can't know what Safinn is about to do , or he'd be taking steps to protect Spectre, not poncing around with a disembodied eye at a jazz party.
So what is the secret?