Your narrative comprehension is blinded by your low effort powerscaling analysis.
By the way, Saturn > Kizaru whether you like it or not.
Luffy will use Gear 5 against Saturn, whether you like it or not.
Nika is definitely going to appear in front of Kuma and Bonney.
Saturn is a "spider" so Luffy will stomp him with his foot, with an attack similar to "Gum Gum Axe" that Kuma saw Luffy training.
Saturn keeps calling humans insects so being crushed like one will be very satisfying.
Also, in Hindu mythology, Lord Saturn (Shani) is associated with Blue Sapphire.
In the same mythology, Lord Saturn (Shani) lies under Bajrang Bali's feet (Hanuman's feet).
Luffy will use "Bajrang Stamp" against Saturn and crush him like a spider, making him lie under his feet.
There is no other possibility, it is the most fitting shit one could come.