Just a thought that occured to me. What would you think of Feywild and/or the Shadowfell being fleshed out into full campaign settings. Something that could be used to run games completely set in those places, not unlike the FRCG. Even if you didn't want to use them as full settings it would great supplemental material.
I'd buy it. Personally, i'd prefer that the feywild, shadowfell, astral dominions, and elemental chaos each get their own book with locations, adventure hooks, some example adventures, and npc's.
Feywild could give us the isle of dread, primal stuff, faeries and giants, and slant towards 'wilderness adventuring'.
shadowfell could give us ravenloft, undead stuff, and slant towards 'city adventuring'.
astral dominions could give us spelljamming, divine stuff, githyanki, githzerai, angels, demons, and gods, and slant towards 'planar travel based adventuring'.
I don't know what elemental chaos would give us...
I would prefer not to be flooded with hundreds of new feats, paragon paths, magic items, etc.