It would make exploring the world so much more exciting, if you could encounter rare mobs and NPC’s, that could spawn anywhere in the world, impossible to camp, granting you great loot or offering special services.
The mobs would be tougher than normal, and could have a chance to drop rare items, exotics, special mats, even cash shop items, as well as perhaps a small chance to drop unique cosmetics, and a tiny chance to drop legendary components.
The spawning would be rare, some mobs could even spawn only once, and drop a unique cosmetic item.
It should not affect the game economy in any significant way, but would make the world a lot more interesting to explore
The unexpected is most exciting.