P4 is bringing in an elf. So why not orcs too? P1 isn't even asking to play an orc, just that orcs exist and that there be orc encounters. Firearms are in the DMG and shadow monks are in the PHB.
GOT equivalents...
Orcs = wildlings (misunderstood as all barbarians before characters get to know them better)
Elves = Ancient royal family, very few true elves still exist but lots of people claim a bit of elven blood
Firearms = if there are dragons and fire magic, firearms do not seem far off. Make them a technology from across the narrow sea.
Shadow Monk = Arya Stark and the Faceless Men. That one's easy!
The characters start at Level 5 because they are knights and high ranking people.
None of this actually sacrifices what the DM wants to run, it adds to it. And it gets the players involved!
Then why should they? Why is no considered a fail-state and not a valid option?
Why is saying Yes a fail state and not a valid option?
I don't see these as an either-or argument. Neither the GM nor the player are in the right. Instead, the question is "Does one collaborator's voice count more than others?"
I would argue that not only is the answer a big, strong NO, but the RPG experience is improved when more participants are given more creative power!