I haven’t found any problems with my braking systems before, so I’m wondering where you’re getting this from? As far as seats go, I figured carbon fiber was really stiff and wouldn’t allow a whole lot of flex through the chassis. Can you elaborate on the weights idea though? I would think possibly carbon wheels could work, and like steering columns possibly. My thoughts would be mostly anything in the steering or anything that rotates since rotational inertia is ever harder to overcome than static. Possibly the axle, if you can make them soft enough while still being lighter than what we use now. Regarding the bumpers, the plastic ones already absorb a large amount of energy, and I wouldn’t want bumpers that just disintegrate, especially when most impacts are not with large solid objects. Foam bags and Tec-pro barriers work pretty well in all the accidents I’ve seen.
I have personally not come across the brake issue either but that is just me.
I was thinking more in line with parts like the Steering Column rod, Steering Rods, Pedals, Heel Stops, Seats, possibly Wheels, etc.