Evolutionarily the question is hard to answer because there has to be a survival advantage (or other very rare circumstances) in order for a mutation to proliferate through a species.
Back in the days of explorers, I'd have said magnetoreception. Being able to sense Earth's magnetic field would add considerable survival advantage to anyone at risk of being lost at sea or in the woods. Some bacteria, insects and birds (among other organisms) have this ability already, with homing pigeons probably being the most famous for taking full advantage of this sixth sense. Today there just isn't enough need for such an adaptation in humans.
Here is one idea that would add a survival advantage and would not be an adaption of an already existing sense:
Ultraviolet (UV) detection - this will become more important as the ozone layer continues to be damaged by human-caused emissions, allowing the sun's harmful UV frequencies to penetrate our atmosphere. I have in mind an additional set of receptors only on the face, since it is the area most affected by the sun.