I see 5e as even more open to customization than other editions. Backgrounds, and the ability to make a custom background even for AL play leads me to this conclusion. While customization and reflavoring are not the same, they are very closely related. I see 5e as one of the most open ended editions thus far.
the example you gave of the player in your group reflavoring the sorcerer into a scion, imo is not reflavoring or even reskinning, there is a direct rules change in that his "magic" is not magic and therefore can't be detected or dispelled as such.
It was a hypothetical example (one that came up in other discussions before the mystic was floated), but that was the point, that it was an example of re-skinning inevitably 'touching' the third-rail of mechanics.
I have not taken re-skinning that far in any 5e game I've run. Mostly gear. One character's shortsword, shield & javelins are gladius, scutum, and pilii - that sort of thing, similar to 3e (some examples there: a caster's shield re-skinned as a holy tome, a lance re-skinned as a sarrissa). In 4e, it was out of control (actual I'm-not-making-this-up examples: an umbrella re-skinned as a light shield, a holy symbol re-skinned as a pistol, rods re-skinned as wheel-locks, a genasi re-skinned as a Djinn, a dragon re-skinned as a water elemental, a pixie vampire re-skinned as a sentient mosquito, a Sorcerer re-skinned as a mad scientist with a flamethrower, a druid re-skinned as Dr.jekyl/Mr.hyde, an Avenger and an Ardent re-skinned as 'secret agents,' a warforged warlock re-skinned as the terminator....).
In 5e, it's back under the DM's control. (Like mouseferatu said "
with DM approval.")
I might be using the wrong words or even, be discussing something completely different, but there is a huge difference between reflavoring something, and actually changing any game mechanics involved.
That's a fine, perfectly clear way of saying it, and I don't disagree. It's just that the line between 'flavor' and 'rules' can be very blurry. That's the case in 5e's natural language approach, and it puts re-flavoring, like rule changes, in the DM's court. Which, really, is where any sort of rulings belong.
I feel that 5e is the most open and flexible system, and I feel that reflavoring without changing any rules is encouraged.
DM Empowerment makes 5e enormously flexible and customizable. Work with the DM, and you should be able to play any character you can imagine. But 'reflavoring'
without changing any rules (and thus, theoretically over/under the DM's head/nose) is a lot trickier, because it means teasing flavor from rules text, and, IMHO, working with the DM to customize flavor&rules is the better approach. (In part because they can end up being the same thing, perhaps even unexpectedly.)