I believe the Fahrenheit scale was originally set up for 100° to be human body temperature. We're just built colder now I guess? I had to look up what zero was and apparently he originally set it at the coldest the air had ever been around his village, but later had to standardize it and so cooked up some brine that froze at 0°.

I would propose that 100 should be calibrated around the wet bulb temperature, which I think is around 105°F but varies with humidity. That's the temperature where sweating doesn't cool you off any more, so any temperature 100 or more is deadly to most people. I like 0 being freezing for water, seems sensible and is also a good "prolonged exposure to this or lower will kill you" cutoff point.