Maybe it wasn't, but this post (and a few before and after iirc) felt to me like it was saying it specifically wasn't good enough that the GM went with the player idea - because the GM going with it were showed the GM still had final say. So the GM should let the player tell them what it was with no final say.
Lots of good points from both sides here. And à lot to think about. It also goes back to the perception of what kind of backstories a DM is ready to allow or not to allow. Whenever a player was making a level 1 character on his own with a mile long backstory, I was getting serious hitches as...
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I very well could have been over-reading it. That seems to be a thing all over the place in this thread
I'm not certain which post you meant... was it the one about the player saying his character's sister worked for the mayor?
If so (with the caveat I don't know the entire scenario), I don't see the problem with allowing that, overall. Why not? It introduces a new NPC to the world, and makes a connection between that NPC and other elements of the fiction. The world becomes a bit more detailed, the PC has a connection the GM can use in future situations. There's a clear path forward and you get closer to the actual important bit of meeting with the mayor. The player is actively adding to the world....what's not to like?
My question here is that if the action in the game has pointed the players' characters toward the mayor's office, surely the GM has some ideas about how they can go about interacting with the mayor and his staff, right? Chances are the players would need to make some kind of roll... maybe Investigation or Persuasion or something similar.... to learn some information that they would then use to proceed on with the mayor in some way.
So isn't the role the sister plays in this scenario going to simply be provided by something else anyway? Why not go with the idea of the sister instead of making the players interact with the mayor's personal assistant or whatever first? You can still require rolls to be made. The sister may not have complete information, or she may not want to share what she knows, or risk her job to help. And so on.
The fear, it seems to me, is that these kinds of player-side introduction of fiction are auto-wins... and that once they do it once, then they'll do it again and again, and suddenly you have a PC with 12 sisters all of whom work in important places! But it need not be an auto-win... you simply shift the nature of the challenge; instead of figuring out how to get an audience with the Mayor, you have to figure out how to convince the sister to get you the audience. And it need not descend into absurdity; players absolutely are capable of the kind of discipline that many are reserving only for GMs.
It can simply be a tool for the GM to use that is largely identical to something else he would have used anyway.
Another way to look at it is, what do you accomplish as a GM by saying no? No, your sister does not work at the mayor's office. What does that accomplish? It shoots down an idea that could be good fodder for future situations. It blocks contribution from the player, and may prevent some amount of investment on their part in the gameworld. It preserves the ideas that the GM had already set as the ways to resolve the situation with the mayor's office or else preserves their ability to decide how the situation is to be resolved.
What do you lose by saying yes? Your prepped Mayor's Assistant NPC, who is likely used once and forgotten? The authority to control the narrative in its entirety? What else?