I think the player's adding to the world is a great idea. The game belongs to them as much as to the MC. Adding something they want means they'll be more invested in it, as opposed to the MC trying to guess what they want.
If the player's a twink, and wants to find Excalibur, or a piece of the True Cross - or something equally silly and out of the setting, well first of there should be a social contract in place where the MC frowns and asks, "really?"
If the player persists, well, there's a failsafe. Even if the player rolls 12+, the MC still chooses the outcome. "Excalibur is far away. Really, really far away."
If the request is reasonable - like finding a doctor to save the PC's dying sister, there's a way to do it. And the MC can put the person, place or item square in the path of one of the Fronts.
Therefore, if x exists, the PC will want to go towards it. Complications will arise based on the effect the MC chooses. Then characters will try to overcome. A narrative is weaved, based on the obstacle and the actions of the players.
If anything, this is a Move that helps the story forwards.