Mine is, or was, that Whitebeard is Crocodile's father. I believed that Crocodile had a falling out with Whitebeard very early in life and they both very much disliked each other, resulting in Crocodile trying to defeat Whitebeard once he thought he was strong enough. Also, at the time I had this theory, I was very much interested in the quote where Crocodile and Daz Bones are reading about Luffy heading for the New World and Daz Bones talks about Luffy moving on from Marineford, to which Crocodile responds "are you mocking me? My wounds have already healed", or something along those lines, possibly hinting at what happened to Whitebeard at Marineford. Also, Weeble has come out claiming to be Whitebeard's son, and the stitches/scars on his face look similar to that of Crocodile's, which may be a pointer. Anyway, I did think this was Crocodile's past he didn't want known as he's a proud character and everyone knowing he lost a battle against his father easily would be thoroughly embarrassing for him. How Ivankov would know about this and not anyone else I don't know, and honestly, after looking back at this theory for a long time it's pretty weak and unlikely. However, you said craziest so there ya go.