If ever both party agreed for a rematch, I wonder which weight Division are they fighting, would it be Canelo's division? or Bivol's division? If ever Canelo go up in weight division just to fight Bivols in his natural Division, I think we will be seeing the same result like their first fight.
I don't think that Bivol can go and fight at 168 lbs, he has been a true light heavy weight fighter in his career. So most likely if Canelo insisted at this comfortable weight, Bivol is not going to take the fight. Canelo has 2 fights already in light heavy weight so I guess there is no problem for him moving up again and rematch Bivol. Although the result might still be the same as you have said.
Aside from that, Canelo don't have any power now to insist or demand what he want like to settle and contest their rematch in 168 pounds because he is now chasing Bivol, not the other way around. Furthermore, their first encounter happened in Bivol's division, so it's still better to make it happen in light-heavyweight and just like you guys said, this isn't the first time for Canelo.
Depends on the management as well as the hype for their rematch. Money talks, as they like to say. If audiences will really push it, things might change and that includes fighters' plan. Same thing goes with rematch of other fighters or in other fighting sports; a player (often the winner) could just move on and seek for the next contender but if the result won't satisfy the fans, teams of each fighter could negotiate to feed that demand. And I believe same idea implies on choosing their next opponent especially if they are on the same weight category, or if their names happened to be on mainstream and on the same level and in some instances 'ties', on their names.