Maybe it`s rather boring, but I`ll prefer robotic internal organs, especially controllable ones.
Say, to intentionally slow your heart beat down if you`re too stressed or pump more adrenaline in extreme situations would be nice.
This one is actually very useful and it has so much potential. I think there was recently a news wherein scientists are already creating hearts in laboratories. Creating internal organs is already being researched on and maybe already being done already, in case I am not updated. If they could make hearts that could potentially function forever, that would be amazing. Heck, why stop with the heart? If you could replace every sick part of your body, that would be crazy. lol
Definitely not stop with only the heart.
Robotic blood filters, air filters (instead of lungs may be?), better digestive system that can process food more efficiently immediately come to mind.
Potential brain augmentation, cybernetic eyes with advanced optics, what else?
But all in all, bionic arms and stuff are cool, but what`s the use of them if the body itself is getting older?
You need to have the proper "foundation" in the first place to use the fun stuff without biological limitation, imo.
This may seem far-fetched, but the topic begins by mentioning Jump-Boots.