Prediction: the big secret about Crocodile? Crocodile was originally a woman. Ivankov performed a sex operation and turned Crocodile into a man at her/his request. Character has a lot of feminine features, personality also feels like a woman. Would also explain why Ivankov knows his secret and why s/he is so sensitive about it being revealed.
I thought ivankov was talking about a weakness rather than a secret. Somehow I don't think crocodile is the type to care about such a secret being revealed. That said, if crocodile really had been a woman I would think he'd have some sort of womanish quirk or something right? I don't think he used to be a woman.
Too many people think that if Iva is involved with something, sex changes were involved also. Heck, why isn't there a thread on this stating that Dragon is actually Luffy's mother?
Amazing how a lot of people think they are the first to come up with this idea, but forget that perhaps Crocodile wasn't born with a hook where one hand should be and a hideously unhealed scar across his entire face.
There is obviously plenty of Crocodile backstory available without Oda having to resort to growing breasts on everything.
Also: what features of Crocodile's are feminine? I see none.
I think the sex change idea has merit, but to be honest when I first read that chapter I thought the "secret" had something to do with the hook and scar as well; I thought Ivankov might have been implying he was the one responsible for both. But seeing as there doesn't seem to be much of a grudge between them, I guess that theory's done.
I doubt the sex change thing is the answer, either. Croc has always been portrayed to be an "in control" kind of guy, and for him to be insecure about his gender all along would be out of character in my opinion.