Hmm, cargo pants with an integral belt come to mind. Cotton is not really very practical at temp extremes, but does well for moderate climates. In really hot weather, supplex nylon pants from REI seem to hold up well and are about as effective at keeping your body temp down as you could hope for. For cold weather, wool is still one of my favorites, especially for hunting where synthetics seem to make way too much noise. Rivers West out of Seattle makes a rainproof, windproof, breathable fleece that is darned near indestructible.

Keep in mind that synthetics do not like to be exposed to excessive heat (eg sparks and flame).