With how things have been going, Saturn is the main villain of this arc especially after reading Cinera's thread.
However Kizaru will be Luffy's main fight not Saturn.
Think of it like the WCI arc, Big Mom was the main villain but Luffy's main fight in that arc was Katakuri not Big Mom.
To me Saturn is set up as Kumas ultimate villain. He was there damn near every step of Kumas life from God Valley to his loss of his individuality.
However, the main side characters of the arc are more connected to Kizaru than Saturn
- Trained and found Sentomaru
- Shared his power with Vegapunk
- Was there to monitor Bonneys and Kumas surgeries
It’s hard to picture a main antagonist who’s so far removed from the actual characters of the plot as Saturn is.