That's the newest "Ask a ScienceBlogger" question.

I've got a couple of ideas. Somewhat smaller, bilaterally-symmetrical hearts and livers would be a good start. Complementary kidneys work pretty well, and you can survive with only one. It'd be nice to have backup livers and hearts as well.

I also don't like the fact that we have one entryway for air and food. I'd split those up.

And speaking of division of labor, as a male it'd be nice to have separation of peeing and ejaculation.

But of course there's the brain (and this one is for the ladies too, sort of). Our neocortex has virtually exploded in terms of evolutionary time. One consequence is painful childbirth and encroaching limits on cranium sizes. You see, our heads keep getting bigger and bigger for our bigger and bigger brains, and they still have to fit through a woman's pelvic structures. The neocortex has basically grown by expansion in the horizontal direction, e.g. the surface area of the cortical sheet. This is inefficient, since the limiting factor in most networks is communication, i.e. wiring or cabling, and the neocortex is no exception.

In much smaller mammals, the white matter (which is composed of axons) makes up as little as 7% of their neocortical volume. In humans it's over 40%. Half of our neocortex is basically cabling, and that's because while functionally it's a hierarchy, it's laid out horizontally like a tablecloth.

So if cortical areas were stacked like pancakes, we'd have lower brain volumes, but more efficient function, and more room for expansion. Also, women wouldn't scream as much when they squirted out offspring. Sure, we might look like coneheads, but that'd be kinda cool, huh?