Lets say the DM has done a ton of worldbuilding. He has created the entire pantheon of gods, and their respective religions. But he has also gone beyond that, and also written in depth about the various theologies of these religious groups, and what rites they perform.
Now lets say a player wants to play a priest of one of these faiths. But for his priest he wants to have some say (or 'input' if you will) regarding how his priest exercises his faith. The question of input is thus, can he do that?
If the DM has decided that the priests in his campaign worship all the commonly accepted gods, can an exception be made by the DM for a player who wants to dedicate his priest to one specific god?
If the answer is 'no', then I think we've reached an edge case where the world building gets in the way of a possible shared fiction. If the answer is 'yes', then I feel the player does have some 'input' in regards to the world building.
Let's test this against a particular scenario which I have constructed specifically for the purpose of getting the answers I want.
As the initial seed for the campaign, I asked each player to come up with a person who would be on a boat which ran scenic tours. They came up with various concepts. The skipper of the boat; passengers such as a Science! inventor-professor, a beautiful movie star, a millionaire and his wife, etc.; I was kinda disappointed that no one chose to play a stowaway. Oh well. Anyways, what starts as a three-hour tour becomes a shipwreck, and the story explores their survival on a remote island.
After a while, one of the regulars wants to bring a friend into the group. I agree that one more castaway could end up on the island. The friend wants to play a priest. We discuss the priest's religion.
He wants to play a Protestant priest, and he wants one of the rites to be... uh... something that Protestants don't do, nor Catholics, nor Orthodox, nor Jews, Muslims, Buddhists, etc. I tell him, no, sorry, the rites of *all* those variations are already established, and there is no available design space in that area. He can invent a new religion, and give it whatever rites he wants, but then it won't be a religion that any of the other PCs have ever heard of. Heck, even if he imports Bokononism from "Cat's Cradle", some specifics of that have been nailed down as canon.
So if we play any game with in which the setting starts with the real world, plus the specifics of the PCs and the parts of the world immediately involved in their adventures - cloak and dagger, Westerns, etc. - then have we "reached an edge case where the world building gets in the way of a possible shared fiction"? Because of how thoroughly the real world has already been documented? I mean, look at Wookipedia, or Candlekeep for Forgotten Realms, or Memory Alpha for Star Trek, and all three of those combined don't nail down as many details as Wikipedia.