When I say improvement, I mean adjustments in their gameplans and how they fight. A 35-36 year old won't be something new suddenly. Still, I firmly believe that GGG has the more room to improve in the ring in terms of how he attacks. Canelo, for his own part, can't really change much and asking him to do more of something will be difficult because when GGG is patient and jabbing, he doesn't let his opponent get any traction (hence the significant chunk of the fight in the middle book-ended by the first and last three rounds). Canelo will want to exchange with Golovkin, but can't do that all night, and he will have to take breaks. Every time he takes breaks he loses the rounds. He simply can't do more, he can just be as good as he has already been. He may take more risks and mix it up more, but those things can only happen if he can get past GGG's jab. Canelo took zero risks when GGG was jabbing because he had no way of getting away with an attack because of the Golovkin counter. When he was able to walk inside (look at round 10), then he could get some work done. GGG is slower.
Canelo won't be able to increase his pace, that's the nature of him as a fighter. It always has been. His fast-twich movements and large build will always put a demand on his system. His stamina means he fights in spurts and doesn't put forth much of a sustained effort. That's just his build and body. GGG fights in a more sustained, slow burn, sort of manner.
Notice Canelo only outlanded GGG by 4 power punches (and I saw at least two that Compubox counted that were clear misses, but that's another story). If GGG jabs and then throws to the body, with his right or left uppercut and right uppercut, he can bank more power punches. When he did that, he was being more effective. It was the first three and a last three where his jab was more sporadic and when he was losing rounds more clearly (though not necessarily decisively, it was a good fight).
This is what I mean by more room for improvement. GGG just needs to adjust where he throws and mix up his punch selection (we know that he has shown an ability to do that), and he needs to be more sustained with his jab. Canelo will lose a close decision if that happens because his success came largely when GGG was not jabbing.
We also need to accept that Canelo was used to that kind of fight hype and atmosphere. He walked around the whole time leading up to and during the fight as though he belonged. GGG seemed a bit distracted and absorbed in this atmosphere.
Lastly, and I really saw this by round 3 to 4. Golovkin started to see Canelo's punches better and settled into the fight. His eyes adjusted and that's something that takes a bit if it is new. Canelo was always seeing GGG's punches. Going into the rematch, GGG's eyes won't be taking all that time to adjust again.
This is why I feel that the rematch offers some interesting points. I don't think GGG is a shoe-in to win. In fact, I think the fight is closer than it was the first time. One day GGG's chin might even say "enough's enough." He is 35 going on 36 and I've said this before: a pressure fighter fades earlier in life than a pure boxer. GGG is on borrowed time. A pressure fighter's peak fades after early thirties.
So I can't argue it clearly for one guy or the other, but I do feel that GGG has the more room to improve in how he attacks Canelo. Canelo will offer no surprises, he will just be damn good.
Sorry for the rant lol.