Luffy vs Saturn will 100% happen.
Luffy will be the one to defeat Saturn, anyone who says the opposite or disagrees is coping and doesn't understand narrative at all.
One Piece follows 2 basic rules:
- it is a shonen -> it's a manga for a male audience with a main protagonist
- it follows the damsel in distress narrative device -> Bonney is the damsel in distress that is oppressed by Saturn and Nika is her savior
Regardless of that, Nika is the Warrior of Liberation that will free Bonney (and Kuma).
- Bonney contracted Sapphire Scale from Saturn
- Bonney was still oppressed by Saturn in Egghead (she was held and almost killed)
- Bonney is waiting for Nika through the legend of Nika spread by her dad
Outside of Bonney, the other characters are secondary "damsels in distress".
- Kuma is a slave who was turned into a robot by Saturn, he is also waiting for Nika
- Vegapunk is also waiting for Nika, he is on Kuma's side, he danced to Nika, and he was brutally hit this chapter
More importantly, Kizaru, the admiral who came to Egghead is the perfect damsel in distress:
- Kizaru is only a cog in the machine so he doesn't decide what he does
- Kizaru is conflicted, his justice is unclear
- Kizaru cares for his friends but he has to do his job
- Kizaru obeys to Saturn, no matter if the decision hurts his friends
Bonney, Kuma, Vegapunk, Kizaru and Sentomaru are all waiting for Nika to free them from Saturn, period.
Now, who will fight Saturn?
I think it will be mainly Luffy's fight but I'm expecting to see Bonney land a punch on him with Nika-ish future and maybe some action from the ancient robot.
I don't think that the ancient robot is there to focus on Saturn. Instead, in my eyes, it will most likely replace Vegaforce and carry the Sunny to the coast.
What about Kizaru?
Kizaru is conflicted and his justice is unclear. Kizaru will not betray Saturn before his defeat by Nika, so what about him?
It is possible that Kizaru intervenes to fight Luffy but that's not what will mainly happen.
When the ancient robot carries the Sunny and it tries to escape with the Straw Hats, Kuma, Bonney, Sentomaru and Vegapunk, Saturn will most likely order Kizaru to go after the escaping crew.
Luffy may be unable to go after Kizaru since he'll be focusing on Saturn and he may be preparing his finisher attack. Also, Kizaru travels at the speed of light.
I think that in that instance, Zoro will step in and protect the Straw Hats from Kizaru. Zoro will knock Kizaru out and have his revenge on Sabaody, which will perfectly mirror that moment.
Then, Luffy will defeat Saturn with Bajrang Stamp, crushing him like an insect/spider, and using a stronger variation of gum gum axe, the attack that Kuma saw Luffy train. Which is fitting because in Hindu mythology, Bajrang Bali (Hanuman) steps onto Saturn (Shani) with his mighty feet to control him.