Kumas personal story has been about Kuma becoming a warrior like Nika. He said it to Saturn himself and even uses the hand of liberation as his sole weapon.
Saturns entrance feels like it was designed to hype up the gorosei, the “no one look at them!” The pentagram, all of it was awesome, however, Saturn in the end just feels like another Gorosei.
He really doesn’t feel personal or impactful to luffys journey so despite Saturn being a bigger individual in the world, Kizaru still is the bigger enemy of the straw hats imo. Hell Luffy himself even mentioned two years ago. So the idea that oda would end Luffy vs Kizaru so… unsatisfying, seems wrong.