Tbh Saturn still just exudes this "Raid Boss" type feel, like Oars, that you just dont get from Kizaru. Everyone hits him and he just keeps getting back up not unlike Oars in that way.
I smell a big hype chapter coming from him, but IMO the only answer to Luffy not facing both villains (nobody is taking over his Kizaru fight obviously at this point) is that everyone else ends up facing the Gorosei together with him.
This way you still hype that the Gorosei is one of 5 massive endgame enemies and there are 4 left like him, plus Imu. But you can also have him lose in an arc where Luffy is already distracted by an Admiral.
Just my 2 cents on that, we'll see what happens.