One thing is clear: Saturn is going down here. I agree that its very likely that Luffy will be the one to beat Saturn. It makes perfect sense narratively. Bonney has to see Nika because her faith is wavering. Kuma, a lifelong fan of Nika, needs to see Nika's greatness before dying.
The question is, will this be another Luffy-vs-Kizaru type of fight or a quick defeat for Saturn? My money is on the latter. In that case, Kizaru remains the main dish for this arc.
One possibility Im seeing is that Luffy ends up fighting both Kizaru and Saturn at the same time. A lot of us are left with this question of what Kizaru will do or who Luffy will fight when it may just be as simple as he ends up fighting both. In a way, this was a similar case when he technically fought Hody and Decken at the same time.
Its even gotten me thinking about the Robot. It seemingly activates only when it hears Nika's heartbeat right? If its not soley used as an escape measure, it must be used to help Luffy deal with his enemies no?
In that way, I can definitely see Oda setting up a 2v2 between Kizaru/Saturn and Luffy/Robot. And sure, it might be the "Luffy show", but thats always Oda's MO with every arc anyway.
Idk, this is the best way I can reconcile the sideline of an Admiral with a Gorosei while also escalating the scale of danger from Kaido.
After this, the only real avenue I see is Akainu, Blackbeard and Shanks being involved in an arc or arcs together anyway to up the scale of conflict.