Forgotten Realms being the default setting is... not good. The Realms are their most famous setting, because the realms lends itself to books very easily. Books can have the sort of dramatic, spanning plotlines the Realms needs, while also focusing on the main characters lets you push aside what the "big names" are doing in the Realms. In a way the realms runs into the issue of 'good campaign settings and good novels are two very different things.' (as DM of the Rings points out in a hysterical manner, LotR is a very bad D&D campaign)
Eberron is a tad too... I dunno. Steampunk/high fantasy mashup to really be what drives their marketing. It's a really good setting, but it's not what people think of when they think D&D, so they have to go to another setting for their default.
I love PoL/4E cosmology, but it's not a defined setting, and they said some of their strongest feedback was that players want much more defined settings. That means it's out practically by default.
Dark Sun is a 'kick you in the teeth' setting that hates people, forgives nothing, steals all your magic items, and spits in your face. I love it. It's not for new players. Ravenloft is this taken to 12 - dear gods, most adventures in Ravenloft are "GET OUT OF RAVENLOFT!"
I say reboot Greyhawk, fix up a lot of the problems with it that Living Greyhawk and all the other nonsense that was done to the setting over the years, and make it a neutral, interesting setting with some defined places, some defined countries, and some defined powers. I hope they do what they did with Exalted - define some things, and specifically DON'T define certain things. Like:
"Here are five countries. These are details on three of them. Lots and lots and lots of detail. Here are two countries. Here are some very vague facts about them. Everything else about those two is left for you, the DM, to decide."
It's hard, but that's seriously my best thoughts on the approach they should take.
As for cosmology, do a mashup. Great Wheel, Points of Light, let people pick what they want to run with.