This crossover is more obvious than others because of Luffy and Mr. Fantastic having similar powers, but it turns out that the members of the two teams have some other similarities in their powers too. Nami can turn invisible via her climatact. Sanji's foot can go on fire. And Chopper can transform, though the Thing's transformation was one direction mostly. I was already drawing the picture when my nephew reminded me that Luffy's brother could have been the Human Torch.
This picture took a little longer than I had planned. That's why the date in the stamp said 2011/09/30(which was 2 days from when I started the picture).