Somebody got high and mighty (and put max ranks in diplomacy) and gave a B5 John Sheratan style "Butt Out!" speech to the Gods, (rolled a nat 20) and ,well, they did. They packed up their toys and substaining influences and left. Some of them were nice enough to leave their portfolios on auto pilot (like death, birth, disease, seasons, gravity...little things like that), but some where not (like metal smithing, weapon skills, continuing fertility of patron races, common magics). They also locked off the PMP from all others except the EP, AP, PEP, NEP, Elemental planes, and the Plane of Shadow.
Their patrons spent a great deal of energy reinventing basic metal working, but for the life of them, no one could remember how to make a dagger or an arrow. Also, most of the common races plain forgot to have sex...(their respective matchmaker goddess and fate spinners having left).
Now the last of the elves are 1000+ and are dieing off....as are the dragons. No more goblins, kobolds, humans, dwarves, horses, cows, sheep, dogs, cats, medium vipers....(thank god the MM wasn't too definitive)...solitary bats and rats but no swarms...
You make make and equip characters as follows:
No races, classes, spells (treat as deleted from class list), weapons, feats, templates, magic items, alchemical items, weapon qualities, special materials, monsters (plants included) or animals from PHP, DMG, or MM. These are the exceptions: weapon and armor feats for items not mentioned in those books; Skill feats for skills introduced in other books; you many select Power attack only in if you can take 2 feats at the same time, and power attack is a prerequisite for the other feat, which must have been introduced in an unbanned book.
WoTC products only.
No half races descended from races in prohibited books (1/2 giant, elan, illumian, whisper/forest/wild/deep/aquatic/desert/jungle/ect templated)
GM rules:
As above, but also No Hazards from those sources. No traps or constructs based on or requiring spells from the Core 3.
Monsters from other books retain spell like abilities, but not "Cast spells as".
I think that advanced groups with decent libraries could actually play this. I go so far as to say that they could do so with XPB barred as well. (and make use of Com Psi even)