Are there some areas where it's impossible for fighters to improve?

Boxers do neck exercises but improve stamina and boxing ability ....but basically no

 

AJ lifts weights using his neck mscles also, it`s meant to help absorb power in punches, improving power is very difficult, sometimes it can come down to your build, but there are tactics you can use when punching to add snap to your shots which can add power also becoming better at countering can improve your power because a lot of KO`s come from counter punches as the foe isn`t ready and won`t be riding the shot, that`s why Conor was ineffective with his power v Floyd, all his MMA KO`s come from opponents jumping at him adding power to the shot as Conor counters, but boxers rarely rush in like that, the reason I mention McGregor is that he was laughed at by people during that bout because of the bull about his power before meeting Floyd, he had no power at all with boxing gloves, because MMA fighters push their punches and with MMA gloves which are tiny you can still do damage, for Conor to learn to stop his shots would take ages which answers your question, power is difficult to develop along with the ability to absorb shots, I feel from what I`ve seen and read.

 

Unfortunately no. Durability is something you either have or don’t have. Marion Wilson and Oliver McCall are perfect examples of guys that were blessed with great chins.

 

No there is nothing a fighter can not IMPROVE on

Yes fighters can increase punch resistance,...genetics will limit their potential to a point but improve yes....
focus exercises
chewing tough gum
neck exercises
good fitting mouthguard
attitude

 

Neck training helping your chin is a myth imo. A chin is something you're born with, people who had granite chins like Marciano, Ali, Chuvalo, Foreman, LaMotta, etc didn't do neck exercises. (Though to be fair they were born with above average sized necks, and I've never seen a pencil neck take a punch too well)

Many fighters who do neck exercises still have average chins (Tyson, AJ), a little bit above average (Dempsey), or terrible chins (Khan)

Basically what I'm saying is that there's no correlation between neck training and having a good chin.

Going back to the original topic, most boxing stuff can be improved (ring generalship, speed, endurance, heart), even power to a certain extent can be developed with physical development and boxing form/technique, though power is mainly god-given.

A chin is something you're simply born with. You can improve defense, you can improve your will to win/heart, but your brain's resistance to trauma is genetic.

 

So can a chin improve by age, if not by training? Would that explain how it was possible for Cassius Clay to be dropped so heavily by 185Ib Henry Cooper to the point when he was almost out on his feet when he got up yet later on in his career, he was able to stand up to the bombs thrown by the likes of Foreman and Shavers without falling? Maybe it was just a once in a lifetime shot by Cooper but it did kind of add to the impression that the older, later 1970s incarnation of Ali did seem a lot more sturdier than in his younger days.

 

"Chin" probably can't be improved much. General hand-eye coordination can't be improved at all (unless you're recovering from a neurological injury that impaired your baseline functioning). This is why speed bags are so ridiculous. They do nothing but burn calories.

 

I honestly think the Cooper case suggests a very different conclusion, and one so obvious I won't even bother to state it.
I doubt there exists a human on Earth who could just take a perfectly landed left hook from a world class fighter just under 190 pounds, without being visibly phased. I cannot for the life of me see where people get this apparent notion that 185 pound fighters can't hurt ones over ~225 with single shots.
It certainly isn't from direct empirical (or historical) evidence.

 

Neck exercises help with absorbing punches that are straight on like ones and twos but they don't do much to help hooks which typically land on the side of the head or face and spin the head.

As far as the OP's original question. Certain aspects are difficult or hard for fighters to improve whether it be footwork or certain punches. Example would be Joe Frazier who had a murderous left hook but didn't have much of a straight right hand and never really developed one. Rocky Marciano never really improved his footwork throughout his career. He was able to overcome his bad footwork with his brutal punching power and will to win.

 

I would add Lyle, Liston, and Mac Foster to that duo, but this seems pretty sound to me overall.

 

Killer instinct

 

Punching power is the easiest thing to improve in the gym. It may not be the biggest myth, but "punchers are born not made" is way up there. I attribute that to two things: first, trainers said it to avoid revealing their secrets, but now most don't know how to teach it.

Ability to take a punch can be improved by understanding that there are a lot of ways to get hit. Taking clean shots is the worst way.