He declared war of the World Government just because they took Nami, same for Big Mom. I guess it could be confidence, because of the whole D thing, but it just seemed to me that he was behaving as if he already knew the answer (regarding Rayleigh). I understand the likelihood of the Luffy = Roger thing is low, but it does explain a few things that are otherwise a mystery.
Both can hear the VOAT. Roger had an illness that Oda hasn’t elaborated upon, (This could be because Oda feels like it’s inconsequential to his story), and Luffy has supposedly shortened his lifespan a great deal in exchange for strength and, ironically, health. And the picture of a young Roger looks a great deal like Luffy. I’m not saying that Oda is going that route, a predestination paradox, but I wouldn’t be surprised either if he did.