I am not sure if I like the idea. First as others have stated what will happen to things like the hunts? What will happen if you have a relic that needs fates from certain zones and suddenly the swarm is doing them while newer players might be left in the sand. Also next to hunts and fates would much really change? If people have no reason to do something in the overworld they wont be out there. So maybe you will see a bit more people outside but does this outweight the negatives?

What about log in problems at patch days? What about the instances and stability of the maps? What about laggs and DC and not seeing the monster or players if too many people are at a place? Honestly for me Eureka just showed that SE is not able to truly handle 144 in one place. I dont see them handle much more very well.

Solution: Give us more things to do in the overworld. Treasure chest huntings, more sightseeing points, riddles, more lore to find and maybe some activities that are done there. (Like beast tribe quests) Then you will have more people doing it without the need to create x instaces because the map will otherwise crash or without the laggs, anger and salt it can also create.