@Paperchampion23 I already answered you here.
Do you understand what a Surprise Introduction is?
Name me One Person from Loguetown who expected Dragon to be there?
Answer same question for Whiskey Peak & Robin? Same question for Jaya & Blackbeard?
Long Ring Land & Aokiji? Water 7 & Garp? Sabaody & Kuma? Punk Hazard & Aokiji again?
These Characters are like Guests in their respective Arcs, Unrelated to it's Main Plot.
Saturn/Kizaru don't fit this Description at all, not even close, They are instead the Main Obstacle of the Arc, almost every Chapter was preparing us for them.
And those Characters i mentioned are Stronger than Villains who stand in SH's Way.
As i said, they represent a Warning for Unprecedented Danger, which Oda always uses to Hype the Upcoming Major Plot.