It is not part of this ruleset that ANY and EVERY creature that, conceptually, has a higher AC through the concept of a tough hide MUST realise that concept with exactly +1 to AC, just like leather armour!
No, but it
is established through precedent that wolf-like hide gives a +1 to AC. When the DM is later evaluating a creature, and trying to determine what its AC should be, this is a valuable data point. If a beast has hide which conceptually like a wolf, then that's +1. If it's conceptually like an elephant, then that's worth +3. Every single creature in the book sets precedent for the language by which mechanics and fluff are inter-related.
Really? If I statted up a werecreature with +2 rather than +1 of its AC coming from its tough hide, then I wouldn't be playing D&D 5e anymore?
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Let's say we have a perfectly RAW D&D 5e PC. If we gave ten game designers a pure fluff description of what our PC can do, no game mechanics or class names mentioned, and then asked each game designer to invent 5e game mechanics to match that description, then they would produce ten different sets of game mechanics. They would not magically produce One True Way ten sets of identical mechanics!
Now you're getting it! There is no One True Way that every game setting has to work by. Some settings will have werewolves with a thick hide that gives them +4 AC, and others will have all of their durability tied up into their magical Damage Immunity power. The DM (or setting designer) is free to modify the rules in order to better fit their own specific ideas about how the world should work.
The setting where werewolves have +1 AC from their hide, and immunity to most weapons, is a default world that sets up the specific way in which D&D 5E correlates the werewolf-level of durability into game mechanics. That is the official D&D translation of what their default werewolf looks like. Any given DM is free to give them +4 AC instead, but if they want to use the language precedent for how D&D is supposed to describe such things, then they also need to change the narrative in order to reflect the change in the mechanics.
Going back to the 'lines of demarcation' (yes, we all acknowledge that the game is all about cooperation! We don't disagree on that!), while the DM can always say 'no' to ANY part of any PC, the player can always say 'no' to playing a concept, class, mechanic or fluff that they feel has been changed too far from what they want to play.
If they cannot agree, then there is no game! Both player and DM know that, hence the cooperation.
I've already said that the DM can refuse some element of a player's fluff, and that the DM should have a rational reason for doing so.
I'm glad we're in agreement.
Have you really already established that in your game world the fiend has x, y and z powers but definitely cannot mess with conception....before I told you my fluff? Really? Or did you decide, after I told you the fluff, that one thing the fiend cannot do is mess with conception, just so you feel you have justification for saying 'no', when you could more easily have just gone with it?
No, what you did was invent a reason to be unhappy about it.
That is an unfair assumption on your part. Whenever an uncertainty comes into question, the role of the DM is to be impartial - to try and discern the truth of the world, given what they already know about other truths. Whether this sort of thing could happen in a given world is going to be a judgment call from the DM, and they owe it to everyone there to give it fair consideration. It might be easier to just go with it, but it's doing a disservice to the players, by biasing their judgment based on what they want to happen. If the DM just ruled whichever way would make things easier for them, every time, then there would be no point in playing the game.
Even in games where players use DM created pre-gens, in my experience the players are encouraged to customise said pre-gens.
In my experience, pre-gens are mostly used in one-shot games where the DM isn't significantly invested in world creation.
I simply don't recognise our hobby in terms that players aren't allowed to create their own PC's backstory!
And I don't recognize our hobby in terms that a player can unilaterally impose anything upon the GM. If you want to do something weird with your character - such as any concept which isn't even mentioned as a possibility within the book - then you should consult the GM first, and don't be surprised if they say no. Showing up with the expectation that anything you think of will automatically be accepted is a degree of entitlement which ruins the hobby for the GMs.