I have kind of a theory about what’s going on with the Nentir Vale setting, particularly in regards to Greyhawk and Mystara. Some of this is based on conjecture and theory, not actual knowledge of events, as I began my D&D fandom during the 2e days.

My understanding of the early days of D&D is that it diverged into two paths. Greyhawk would become the world of the AD&D modules, and Mystara would become the world of the basic D&D modules. I suspect that the modules each added onto the known worlds, but that there wasn’t a cohesive world from the beginning.

So why create a new setting, then, when we have 3 settings (Greyhawk, Mystara, and Forgotten Realms) that are fully capable of being the basic D&D world?

I think WotC wanted a fresh start. Consider that WotC wants an easy entry point to D&D, not a setting with 30 years of continuity. I think they wanted a clean slate, so they could work in things like the dragonborn and tieflings without any repercussions from existing fans. Imagine saying that dragonborn were always in Greyhawk!

Ah, but WotC wanted to also include some iconic modules in the new world. This was their chance to integrate them in a way that made more cohesive sense. After all, why go from one setting to another to allow your players to travel between classic dungeons when you could have them all in one setting?

Consider as well that Nentir Vale/Nerath is the world of Dungeons & Dragons. It’s the implied setting, much as Golarion is the implied setting for Pathfnder. Shouldn’t all of the iconic adventure sites of D&D be in the world of Dungeons & Dragons?

I could be totally off-base, but I think it’s a theory worth considering.