Oda revealed how he came up with the Supernovas!

During an interview to One Piece Log 27, Oda talked about the creation of the Eleven Supernovas.

Question: Who have more highlight scenes than you imagined?

Eiichiro Oda: “I have to mention Trafalgar Law. Actually it’s not too much to say that all Supernovas have more highlight scenes than I imagined. I created Supernovas right before the chapter where they got introduced. None of them existed in my plot notebooks before that chapter. I was in desperate situation, worrying, “At this rate, Sabaody arc will not be interesting enough.” Then, inspiration for 9 Supernovas came to my mind unconsciously. I should have reduced the number of Supernovas since they were too many, but after all, I made all of them appear in manga!! I’m really surprised they survive till now since I thought about half of them would drop out of tough New World already.

They are worst!! They are the worst generation, indeed.
I imagined Eustass Kid would become an important character, but… Trafalgar Law!! I didn’t imagine he would become such an important character. Weekly serialization is a wonderland.”