I'd be in favour of something like that: Instead of a core PHB, have a core PHB for each setting they're doing. Either that or a essentials style PHB for the core with just elf, dwarf, human, halfling and a few major classes (Cleric, Fighter, Mage, Thief, maybe Druid), and then setting-specific PHBs for Forgotten Realms (that could have Strongheart Halflings, Tieflings, Sorcerers etc in it), one for Greyhawk, Eberron, etc. GMs that wanted to could say they're accepting races, classes, and subclasses from any book for their campaign, or limit it to a particular one or two. A Dark Sun one with psionics and psionic-related (sub)classes, the Dark Sun races and specific classes, etc, that sort of thing. It could encourage new settings that aren't just the same with sightly different geography, like now - you want a camelid race, some weird winged elves, angels, flesh golems, then go to town, with no monks, dragonborn, etc in there just because you feel like you have to add them because they're in the PHB.