I would extend my thighs a few inches to be a little taller. I would also install a cooking gene!
If you had to replace one of your body parts (external or internal) with a really cool sci-fi robot arm that you could program to do anything you want what body part would you replace?
Can't say. Hehehehehehehehehe!!!!!!!
If I read the question correctly you are saying the body part would be replaced by a robotic arm. For symmetry I would replace my left arm.
So hopefully you mean any cool cybernetic appendage or organ and not just an arm . . . .
I would choose my back. From time to time my lower back flares up . . . Just enough to be annoying.
Nee legs and then a new back
Why replace? I would add it as a tail that would act like a third arm.