Well, let's say that their rematch will indeed happen in 168 lbs. and let's say that Bivol have also agreed to it. The question is, is Canelo Alvarez ready to risk his belts and the undisputed title? I don't think Canelo have the guts to risk his status now because he already know Bivol's capabilities. Remember that supermiddle and lightheavy have only 7 lbs. or 3 mere kilograms of difference. Bivol will surely take that challenge for sure if that's the case, no doubt.
Yeah, Canelo already know he's very struggling when he was fight against Bivol, he wouldn't take risk to lose all of his belts. It will depends on Bivol side, does he's really want to achieve more belts, fame or money? if he want to achieve more belts and fame, he should ask Canelo do a rematch at super middleweight division. If he want to get huge pay check, he will agree do rematch at light heavyweight division.
Personally I'd more interested to see the rematch at super middleweight division since Bivol already risk his belt before, next should be Canelo turn.