3e had Greyhawk as the default setting. 4e had "points of light".
In the 2e and 3e days, "the Great Wheel" was the default cosmology with several products referencing it.
Should 5e have a default setting or cosmology?
I personally don't want a default setting or cosmology. I think the rules should avoid mentioning anything other than the bare basics of cosmology. (e.g. The spell "Contact Other Plane" would imply the existence of other planes, but doesn't say what the planes would be.) That would encourage DMs to design any setting or cosmology they want without having feel like they have to shoehorn them into the "official" D&D setting.
What do you think?
I don't want a default either, really, but I'm not who D&D has to market to in order to gain money and broaden the audience: I'll buy a new edition's "core" product regardless, but I'll be picky about supplements based on my own, well-developed tastes over the past 25+ years.
Based on that, I think that they indeed *should* have a default setting, and it should be Faerun. I mean, that's their money-maker, with video games, best-selling novels, and everything else tied to it. Give D&D a strong brand image that helps sell the game, and you've got win...but they have to make that image LOOK REALLY GOOD, so Forgotten Realms needs to look different than WoW visually, needs to set itself apart from Tolkien, and needs to scream "Adventures Await Those Who Enter."
My other tangential point would to keep cosmology out of the core product(s), though. Planar adventuring is great, but doesn't need to be in the first book right from the get-go, and therefore I don't think the typical buyer of a new edition (or the first-time buyer of a D&D product) gives too cares as to what Plane Moradin resides in, or whatever. Cosmology can be revealed in a Manual of the Planes type book, a Planescape type setting, or through DDI, and should probably default to higher-level play, though -- especially if Planescape is coming back -- it should have plenty of supplemental material that works for all levels.
And bring back the Factions, if you do that! Post-Faction War sucks.