Grainger
Explorer
As Pemerton mentioned above, for me it becomes a "say yes or roll the dice" situation. I almost never have anything pre-determined like that. I want to be surprised as a DM and allowing either the players or the dice to drive the campaign allows for that. Since there is no contradiction from the player's point of view, I don't really see a problem. I'll rewrite entire swaths of my plans based on that one addition if I need to.
But, I think as a DM, it helps to build a lot more flexibility into the campaign. Is it absolutely necessary that that specific ruler is opposed to the players? Why? Why not simply shift "A noble opposes your efforts" a couple of steps to the left and instead of Baron Von Turkeywaddle being opposed, now Marquis De Gravy is. From the player's perspective, nothing has changed. They don't know an of that and you get to keep 99% of the prep that you've done. Or, maybe that becomes a "Yes,and" situation. Sure, that ruler will help you, but the important people in his court oppose the PC's, so, now the PC's have to convince the court for help.
Note, this is something of an odd example too, since the players have never actually had any dealings with this NPC, it would be very odd for a player to declare what the intentions of that NPC should be. By and large, you don't make declarations in a vacuum, you should have something connecting the dots. Particularly when trying to declare future events. Adding a magic item into a setting based on the back story of an existing item isn't too much of a stretch, IMO, but, declaring the reactions of an unknown NPC is something I've never seen players do. They want to be surprised too.
I really, really shy away from having pre-determined results in PC/NPC interactions. Unless the interactions either have no real consequences (who really cares what the bartender says to you?) or the results are very obvious (or bloody well should be) to everyone at the table (yes, walking out of a ball held in your honour by the king (or in this case queen) is going to have negative repercussions on your interactions with the queen down the line) I'm perfectly willing to let the PC's and the dice drive the car.
That's a very different style from me. I pre-plan ahead like crazy - not outcomes of PC actions, but the world in which they live. Anyway, I'll give your style of play some thought.