This idea stems from part of a D&D cartoon episode (not really explored there but watching it reminded me) and one of my brother's favorite dungeon locale ideas.
My brother was really interested in created a dungeon called Magnet Mountain. As the name suggests, the mountain would contain a large iron core that was magnetized, and the deeper you went into the dungeon, the stronger the magnetic attraction. This would make fighting with metal weapons difficult and just walking in metal armor very difficult. He mostly meant it as a surprise/difficulty rather than as I intend it below.
I anticipate making certain areas of Trokair difficult, if not impossible, to explore with certain equipment or as a certain class as well as areas that require specific measures in order to survive the environment. Examples include the abovementioned Magnet Mountain, areas inhabited by copious rust monsters (again, no metal equipment), an aerie or pass blustered by constant wind (no ranged attacks, difficult to stay standing), an enchanted glade that will not suffer one to tread the grass (cannot touch the ground), a tomb wherein nothing may breathe (poisonous gas or a magical curse), and a wizard's tower containing an idol that devours magic (no spells or magic items).
This would feed into player-driven research of these mythical areas as well as diversifying parties (as the original Western Marches campaign had a flexible player base with different people showing up to each session).