I like @Armoredsaints idea, as I assumed once you are 1st sent to you next trader "to open trade routes" it would up your quest level once there Sorta like how most MMO have leveled areas and you graduate to harder "zones". Zones being biomes. My ideal setting would be 1 trader in the biggest city per biome and each trader was different. The quests would level based on the biome you are + game stage vs. tier.
Tier is what is attached to the POI if I am not mistaken and how they associate POI and quest tiers.
When you think about it the majority of large infrastructure/factories/sky scrapers would be located in the wasteland biome, as any nukes would have targeted the largest cities. That would make sense in regards to tier/poi/biome relationships.