I would love to see a revamp of the the PHB... sort of a 3.6 version. A PHB that consolidates the most worthy of the the Races, Classes, Feats, Skill Descriptions (for those new uses that were thought up), and other stuff found in the various Complete Books, Races Books, Unearthed Arcana and other books. Essentially the stuff to get a character built.
Then, separately, a complete book of spells (as this takes up the most space).
I would also be up for Campaigned flavored Core Books. A complete PHB just specific to Eberron or to the FR or Dragonlance, or other settings. These books would have redundant information if compared to each other or to the (Greyhawk Flavored) original core books. As a side note - it would be good marketing because WOTC could sell essentially the same base material multiple times to those who might play in multiple published campaigns.