Helldritch
Hero
Anything that would interfere with the reached consensus, anything that simply shuts off an other player's already accepted idea or an entirely outlandish background. Of course, I have seen outlandish background that were so good that we litterally modified a campaign world.I was trying to. There was a lot of complaining that players are railroading GM creative agendas and very little (none really) about how player agendas are being included by GMs. How much can a player propose before it's too much?
Greyhawk have no dragonborns, usually. BUT
One of my players really likes dragonborns. He imagined a whole tribe fleeing a world invaded by orcs and landing on the Abbor Alz. His background story was so good that me and the other players (5 of them) started to imagine and write down the origin of the tribe with the player in question. Now, we have dragonborn in the Abbor Alz and that tribe is slowly growing and it has diplomatic relation with the Duchy of Urnts and Greyhawk. Session zero lasted 4 hours alone with us writing down the tribe's history and leaders and hopes and fears.
So yes, from an outside the table appearance, I am a wee bit restraining players choice. But when something good really come up. I can recognize it and will encourage it. We much prefer to see the story develop from play, but sometimes, an idea is so good that the whole table will accept it and a world is changed accordingly.