Assuming he doesn't face any ill effects of the weight cut and fights at 152 right? If so then Mayweather of course, Canelo's biggest "improvement" is him able to fight 12 rounds on front foot pressuring the other guy without gassing like he used to do at 154 because he picks his shots and doesn't go for 5-6 punch combos like he used to do before. Against someone gunshy like Callum Smith or a shot Kovalev this works quite well for the entirety of the fight because he can dictate the pace but when there's resistance then the problems begin to show up in the second half like the second GGG fight . Someone like Maidana with a better gas tank will always pose more problem to Mayweather than Canelo .
There's no way he can cut to 152 anmore without losing a ton of muscle and very likely he'd be so badly affected by it that Floyd pitter patters his way to another win. Drag Floyd up to 160 (and I'm not even sure Canelo can safely make that anymore, tbh) and Canelo breaks him like a twig.
Mayweather won the first fight EIGHT YEARS AGO in a fairly clear manner. Today, Floyd is a retired 44-year-old with hair plugs and Canelo is a 31-year-old in his prime. it's a one or two-round fight. I'd lean towards one. Canelo knocks him out like Liston destroyed Patterson. If Canelo fought 2013 Mayweather, he'd stop him in six.
Canelo can't even make 160 anymore and Mayweather can't make that weight weight either. Doin so will horribly hinder his speed. Canelo wins in that instance. Lets make it fair and hypothetically make both the same size. Mayweather wins again
Two primadonnas fighting it. Fight happens as two tries bribe judges more than the other one did, in the end they both pay their purses and all their bank accounts to Vegas judges, judges end up in the top of Fortune 500 next year.