This is backwards.
If what you want is a class option that is evocative of a setting, forcing the setting to conform to an outsider's idea of what it should be is actually erasing what the setting actually is. It's giving colonial vibes.
“Colonial vibes” is a little hyperbolic, don’t you think?
There's a thing in FR called a purple dragon knight. It is cool on its own - an elite military commander, belonging to an order of legendary dragon-slayers (the purple dragon in question was an ancient black dragon whose scales turned violet).
Thats fine, but doesn’t need to be a specific subclass. What stops any fighter, paladin or even ranger from being part of this order? What’s so special about this order that would need a unique subclass to realize?
It has a good story and has even had some pretty interesting mechanics over the years, driven by realizing ther fantasy of an elite military commander. That's a good fantasy, and one that 2024 Fighters could certainly be better at realizing.
I mean, I’m all for finally getting a proper martial leader in 5e. I’m not all for the martial leader being tied to such a hyper-specific narrative. Give us a proper Warlord or Marshall class and describe the purple dragon knights as an example of that class, don’t say everyone who wants to play a martial leader has to be from Cormyr and part of this specific military group.
It's not a dragon-tamer. That's not a purple dragon knight. That's something different.
But does that something different warrant a unique subclass, and if so, should that unique subclass be limited to that specific narrative?
If the purple dragon knight isn't cool enough to be a subclass based on its own lore, then don't use the name. Don't gut the concept and pretend it's an upgrade.
This I think is an issue with the original SCAG having made a full subclass based on such a specific piece of realms lore. The more generic “banneret” name they gave it for non-realms settings should have just been its name in the first place, and the purple dragon knights should have been listed as an example of a group of bannerets that exists in the setting. But that ship sailed long ago, and people who thought “purple dragon knight” sounded cool and were disappointed it had nothing to do with purple dragons still want the subclass they thought the name was promising.
What I want out of a PDK subclass is a Fighter who makes a good military commander - inspiring, supportive, granting courage and hope enough to turn the tide of a battle.
Again, though, why should this be limited to such a small slice of realmslore? I’m in favor of a proper warlord, but it should be broad enough to be used in any setting.