User blog comment:AYET/Minute Details Planning by Oda/@comment-1595565-20130628045830

It's really hard to say. With some stuff like the admirals we have no idea if he came up with them at the beginning of the series or if he came up with them a few months before Aokiji was introduced. With the fishmen, it wasn't until we got to Sabaody that we learned about the discrimination by humans. Before that, the only thing we knew about the fishman-human relationship was that Arlong hated humans. I agree, he does do a lot of planning like that, but with some stuff you can't really place the idea until you ask him.