I wish the process of separation of food into useful and waste didn't take place inside me. There should be a process where the food could be digested, outside the body. So humans will be eating just the useful part from food and this would get directly absorbed into the blood. The waste would have been in advance separated (and returned back to Earth may be). The human body evolves to let go of the separation and waste-removal process, just like it is supposed that we lost the tails we no longer began to need.
I wish there is a toilet-free world.