Luffy stomping Saturn by the end of the arc would be mad fitting
Not only because it’s the attack kuma saw him practicing but Saturn is deadass just a big bug lol
Luffy first used Gomu Gomu no Battle Axe to defeat Arlong, a Fishman who hated the WG because his people had been enslaved and discriminated by them. In doing so, Luffy destroyed Arlong Park, which was modeled after Sabaody Park out of Arlong's hatred and jealousy.
Sabaody was the arc where Luffy punched a Celestial Dragon, the people Arlong despise most, an act Kuma witnessed, before Kuma saved the crew from the resulting assault by Kizaru.
Now, cut to Egghead. Like you said, what attack do we see Luffy practicing? Gomu Gomu no Battle Axe.
Saturn: "Nika... you too, are nothing more than an insect!"
Luffy: "CALL ME WHATEVER YOU WANT! YOU'RE THE ONE WHO'S GOING TO GET SQUASHED! GOMU GOMU NO!"
Luffy's leg stretches so far into the sky his foot becomes lost in the sun. Saturn tries to stomp on Luffy, but Luffy holds his ground. Luffy's leg comes flying down, coated in not just fire but light itself.
"DAWN AAAAAAAAXE!"
Luffy drives Saturn deep into Egghead, splitting the island apart.
Narrator: "On this day, the Yonkou known as 'Monkey D. Luffy'... defeated the Gorosei Jaygarcia Saturn. The results of this incident would spark the weakest in the world to rise up, and oppose the World Government that had ruled over them for so long. In that moment, a father and daughter, who had dreamt so long of living together in a world they were told could not exist, saw the truth which had been hidden for hundreds of years. Nika and the Dawn were real, and they had begun to rise."