The scenario after Casimero vs Inoue will be:
If Inoue wins, Casimero will ask for a rematch.
If Casimero wins, Inoue will ask for a rematch.
Then regardless of the result:
Donaire will challenge Casimero and Inoue.
In the end, if Naoya finished the 2 PH boxers successfully without losing, he will now climb up the ladder and the belt will be left to top contenders at that division.
If Inoue wins, I doubt he will give a rematch to Casimero. His next fight will be against Donaire, and if he again wins the 2nd time around, I guess he would just move to a new division so he can continue his journey in chasing the pound for found king.
Inoue is currently rank at the number 2, https://www.ringtv.com/ratings/
Canelo is sitting at the number 1, well deserved for him but Inoue could go up in weight and hopefully he would be able to bring the speed and power.