Crocodile's big secret with Ivankov in One Piece is yet to be revealed, but a fan explanation suggests a good alternative to the Croco-mom theory.
We already know that the crocodile is Luffy's mother
Crocodile is the overwhelming antagonist of the Alabasta Saga and is one of the villains who came closest to defeating Luffy and the Straw Hats. Everyone thought Crocodile's appearance would end after the Alabasta Saga, but Eiichiro Oda brought him back in the Impel Down Arc of One Piece.
The former Shichibukai was also intimidated by Ivankov, due to a big secret between the two that One Piece fans are yet to know. Many theories explain that the secret is that Ivankov helps Crocodile change gender after giving birth to Luffy. But the truth can be something completely different.
Volume 78 of SBS held a Q&A where Eiichiro Oda answered questions from One Piece fans. In one of the panels, Oda commented:
