I quoted the passage upthread, from Gygax's PHB, in which he flags player authorship of PC backstories, with no consraints suggested on when that might take place.II'm really not seeing where that's supported anywhere in the books, though. Even when the DM just starts with a dungeon and a town, it goes on to explain how the DM is the one responsible for building the rest of the world as necessary, without suggesting that the other players get any say in the matter.
There are well-known passages in the 4e DMG and DMG2 which suggest the possiblity of player authorship (eg sidebars on p 28 and 21 of the DMG and DMG2 respectively; pp 16-19 of the DMG2 under the heading "cooperative worldbuilding").
From Gygax's DMG, p 93 (under the heading "Territory Development by Player Characters"):
You [the GM] must have a large scale map which show areas where this is possible . . . The exact culture and society of this area is up to you. . . .
Assume that the player in question decides that he will set up a stronghold about 100 miles from a border town, choosing an area of wooded hills as the general site. He then asks you if there is a place where he can build a small concentric castle on a high bluff overlooking a river. Unles this is totally foreing to the area, you inform him that he can do so.
Assume that the player in question decides that he will set up a stronghold about 100 miles from a border town, choosing an area of wooded hills as the general site. He then asks you if there is a place where he can build a small concentric castle on a high bluff overlooking a river. Unles this is totally foreing to the area, you inform him that he can do so.
This is quite analogous to the "boxes in an alley" example, except at the terrain/geography level rather than the "urban junk" level.
Gygax's PHB (p 27), in discussing thieves, says:
Any thief character of 10th or greater level may use his small castle type building to set up a headquarters for a gang of theives . . . [T]his will bring the enmity of the local Thieves Guild, and they will struggle to do away with the rival organization.
Not unlike [MENTION=22779]Hussar[/MENTION]'s example of the paladin calling for a warhorse, this is an example in which the GM's worldbuilding and provision of antagonists is expected to follow a lead set by a player.
These are all examples where the rulebooks do not assume that the players have no role in th game, and in contributing to the shared fiction, except playing their PCs within the constraints of those PCs' imagined causal powers.