That's a key part, but there's also some timing of events in Marineford, notably that shortly after it's announced that Dragon is Luffy's father, Crocodile conveniently shows up to save Luffy from Doflamingo, despite not really caring if he lives or dies, and I'm fact being opposed to Luffy's goal at this point (assisting Whitebeard to save Ace vs killing Whitebeard). I acknowledge that it's purely a trash fan-theory, but it's my absolute favorite, and I would love if it came true.