Thousand Sunny being waged against maybe the mystery egg from Orojackson? At the brink of hatching? The last nakama is a mythical beast? Also let's create 10 separated mini-islands in which each SH vs RH happens. A mountain/island contains entirely of a giant labyrinth full of traps for Usopp vs Yasopp, an isolated island the only building at which is a sacred temple or shrine left by an ancient swordmaster to witness Zoro vs Mihawk (of he's gonna allied with RHP at that point)--
--and finally, the top of the highest mountain in the world, even surpassing Red Line or Sky Islands. At the top of that mountain is a peaceful meadow, perfect for Luffy vs Shanks aerial Conqueror battle! The decisive battle to be the one with the most freedom! It'll be soo fun and satisfying.