Re: If you could redo or add something to the human body...
Lets start from the top:
Hair. Going bald is something that happens because evolution doesn't care what happens after you stop reproducing. Not difficult to fix if we're redesigning the human body with an actual design. Some chemical trigger to make hair start or stop growing would probably be nice as well. Like, drink a particular tea and you don't have to shave ever again... but if you want that hair to start growing again, just drink a different tea. Bonus points for separating the hair on your head from the rest of it.
Eyes. The optical nerve in vertebrate eyes is on the wrong side, creating a blind spot. The squid eye has its optical nerve in a better place, and no blind spot. While we're at it, the change to allow low-light vision isn't big - just a reflective coating. And if we're redesigning the eye intelligently, I don't think it's unreasonable to add a fourth cone type and re-arrange the frequency response of the others to give us the ability to see into the infrared and ultraviolet. (This would cause some issues with other colors and would certainly mean every monitor/television in the world needs to be replaced, but that's a small price to pay.)
Nose. I had problems with nosebleeds as a kid, and just avoiding major blood vessels there would be a big help. But if we're here anyway, we certainly can look at certain other animals to greatly enhance humanity's faculty for smells. The ties between taste and smell would give us some automatic benefits there as well, but we can toss in separate taste enhancements if we wish.
Moving slightly further down, lets correct the placement of the
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which takes a lengthy detour because of the different placement of various organs in our early ancestors. Correcting it would fix a number of seemingly unrelated issues.
Then there are a few digestive issues. Lose the appendix so it can't burst, and while we're here it would be nice if our digestive track itself provided all the benefits currently given by the bacteria that colonize our digestive track, to get rid of all the problems people have when some are missing for whatever reason. Remove our dependency on *any* bacteria, and we can strengthen the immune system in a way that would have killed us before. That's a fairly complicated change for a relatively minor benefit, though, so stick this low on the list.
I'll leave changes to the groinal area to others - I'm sure we'll get plenty of suggestions here, even if the majority of posts don't focus on it. I'll just add that being able to give birth without tearing anything important would be nice.
On to the knees. Any good structural engineer will be able to point out how tendons could be rearranged in our knees to reverse some of the harm we did by starting to walk upright. The ACL in particular is prone to tearing with disastrous consequences, and the fix isn't very complicated.
Which brings us to the feet. Feet are actually pretty good, but there are certain genetic tendencies towards flat feet and similar that should be purged while we're down here. And toenails cause more problems then they solve, especially if we're wearing shoes. At least we use our fingernails from time to time.
Next up, a more general issue. We could probably rid ourselves of obesity by making fat production a self-limiting operation - make fat release some chemical that in sufficient quantities prevents the creation of more fat, and ideally makes a person feel hyper so that they'll burn more calories. This also allows us to leave the ability to have reasonable reserves of fat in the case of an apocalypse or something that makes us regret the inability to store energy. This is the first one that isn't either *really* simple or borrowed from other animals, but it's at least *fairly* simple. I mean, compared to the cascades of proteins that trigger the immune system...
Finally, our bones. Bird bones have a much better strength/weight ratio, and while outright switching to their structure would weaken our bones, I think we could take some hints about latices and similar to strengthen our own bones at little cost.
Anything are further things I might suggest. Blood that dries instantly in contact with the air would be better then bleeding out. We could use supplements of iron, titanium, or even exotic carbon structures to greatly enhance the strength of bones, tendons, and perhaps even skin. We're starting to learn enough about the brain to consider ways it could be improved. We might program our immune system to detect cancer and autoimmune diseases in a robust way.
But these are straying into the realm of fantasy or at least science fiction, whereas all the other things are just adapting things from other animals, or correcting simple flaws that arise from our evolutionary history.