Let's take a step back. Because in this last post I think we're really saying very similar things.
I never said that only one reality can have a particular mechanical reflection. I said that any particular mechanic exists because it is a reflection of the reality, and not the other way around. There are more objects within the game reality than there are simple mechanical ways to reflect them within the mechanical language of the game; some of those objects are going to have the same reflection.
A battleaxe and a longsword both deal 1d8 slashing damage, because both of those realities generate the same reflection. That doesn't mean you can take the 1d8 slashing damage and just declare that it comes from anything you feel like; you need to evaluate it for what it actually is within the reality of the game world, and determine the reflection based on that reality, in order for it to have any meaning whatsoever. It's entirely possible a two-handed club would have very similar mechanics to a maul, not because the fluff doesn't matter, but simply because that is the most accurate way to reflect that reality within the game world.
This is where I'm coming from - reskining isn't changing the mechanics, so it only makes sense where the mechanics will have the same expression. It's just changing the description to something else that can be described in the same way - but because the mechanics are a rough simulation, "same way" is pretty broad.
No one is saying to take Eldritch Blast and reskin it as a piece of defensive equipment - the mechanics simply aren't there. But at-will multiple ranged force attacks could be described in multiple ways and still be 100% true to the mechanics we have. Maybe someone calls the "blows of the demon" based on her pact, and someone else describes them as telekinetic blasts because of the mental theme of the Great Old One.
If Alice's barbarian rages and screams and has spittle coming out of his mouth and Bob's enters into a passionless killing focused fury, those are both valid ways to describe it without changing the mechanics. If Charlie is playing in Eberron and want to describe it as his aggressive shifter nature (true-breeding lycanthropes without a curse but that can change), why does the non-mechanical addition of hair sprouting out which is not described by the mechanics suddenly a deal breaker?
Reskinning needs to keep to the mechanics, that the first rule because you are trying to find a way to get your narrative and mechanics to mesh up. So let's start from there as a definition:
reskinning is finding a different description fit by the same mechanics.
The druid class is intended to cover only members of a specific type of organization which teaches its members very specific abilities; if your character concept isn't that you were a member of this type of organization and learned these very specific abilities, then you aren't a druid, and re-skinning the existing class as something else entirely would be a disingenuous way of representing that other thing.
This is untrue because there is more to the world then just the PCs. First example for the druid that pops to mind is I've run treant NPCs that were absolutely, word-of-DM druids. That were not built with the druid class. They couldn't wild shape. They shared some casting and had other features that made sense for a plant-based druid. But they absolutely were still druids, and my players had no problems understanding it. What "Druid" means in-game is
not the same as the druid class.
Now, the druid class, because it is such a specific blend of features, probably isn't a great one to reskin. Reskinning only works if all the mechanics fit. But there's still options. A hermit with a ranger-like connection to nature but without the rest of the martial chassis could have one level of druid (pre-wild shape and circle) and some levels of monk. A hedge witch who dabbles in several types of magic and transforms herself into a cat or a raven could multiclass between druid and some other caster class. See, there are things that make sense with the mechanics, and they don't dismantle the idea of druids for the player because they are never presented as druids.
The fighter is a great example the other way - it covers so many different martial superiority archetypes, with so many different way of applying them mechanically (STR based, finesse, ranged, protection, even before subclasses). If it had fluff it would be ripe for reskinning. As it is, I will tell you with 100% certainty that players have taken the Samurai subclass from XGtE and have reskinned it to have a non-oriental theme. Probably many are noble- or knight- based, or some other ideal that well explains the bonus to to Persuasion given at 7th. As you can guess, I don't see any problems with that.
So, to sum up:
- Reskinning is finding descriptions that match the mechanics.
- Having things that aren't [whatever shorthand like Druid] won't confuse your players if they obviously aren't [Druid] because they are described differently.
- Having things that ARE [Druid] that have different abilities but are described as part of it [like treant druids] will also not confuse players.