I would probably have some evil that takes hold of a land that is challenging for players to defeat. The only way to really claim the land back is for particular players to journey and accomplish trials in multiple places and in the end assemble a weapon that is key in defeating them. And/or open a shortcut through defences to get to the evils source. And/or rally npc ally race or connect area/tribe/duchy to area that greatly improves chance of eliminating foes.