The game is going to have some base assumptions on genre, which could be described as a "setting," which IMO is the only type of core setting they should use. To create a new setting as core is to immediately shoehorn new DMs into that setting, rather than encouraging them to make their own. This was a huge complaint with the Starter Set of 5E, with everyone crying that "Forgotten Realms is the core setting!" The worst thing that happened to Greyhawk was becoming the "core setting" of 3E, because everything unique about the setting was ignored, generating the feeling that Greyhawk is just "vanilla." While I'm not a fan, Nentir Vale/Points of Light of 4E was a unique setting designed to go with the edition, lacking any existing baggage, which I think is the only other acceptable option for me.