Fizban's Treasury of Dragons has two draconic-themed subclasses. These also happen to both be for classes that haven't gotten any new subclasses outside of rules-expansion books (with the exception of the two for Monks out of SCAG, which is almost one of those and also not very good).
Rangers have a bit of a poor reputation in 5th Edition, though as I suggested in my subclass review, the real power of the class is in its subclasses - the two in the PHB were not sufficiently powerful to make the class good, but subsequent efforts have introduced some concepts and mechanics with a lot of potential in them. I'm a bit skeptical about the rework of the core class in Tasha's (while it does a lot to make Rangers more versatile, with features that are pretty much always usable, the power level isn't particularly increased) the subclasses in Xanathar's and Tasha's at least seem to me to help make the class competitive.
The Drakewarden takes a concept that didn't work very well in the PHB, but which got improved in Tasha's, and makes a new subclass that could potentially be quite good. Let's review:
As a Drakewarden, you've formed a deep, mystical bond with a drake - a sort of beast-like version of a dragon (though it's still the dragon type). The drake is your companion and ally in combat, and the subclass revolves around the drake's ability to fight for you and the synergies you develop with it.
At 3rd level you get Draconic Gift: this first grants you the Thaumaturgy cantrip, which is a ranger spell for you. You also learn to speak, read, and write Draconic, or another language of your choice (oddly you could be a Drakewarden who doesn't speak draconic, but the choice option I assume is in case you already know it).
These are mostly flavorful, but obviously both give you some of a dragon's gravitas.
Also at 3, and at the core of the subclass, you get Drake Companion.
As an action, you can magically summon your drake within 30 feet of you. It's friendly to you and obeys your commands. Like a lot of combat pets, it goes right after your turn and can move and use its reaction on its own, but will only take the dodge action unless you use a bonus action to command it, and if you're incapacitated, it doesn't have to wait for your command.
If it's reduced to 0 hit points, or if you summon it again, or if you die, it vanishes. You can summon it once per long rest for free, and you can summon it again if you expend a spell slot of 1st level or higher.
So, broadly, this is pretty similar to the Battle Smith's Steel Defender. Your drake will never be permanently dead, and you can get it back with ease. Works great.
But now, let's talk about its stat block.
The drake has an AC of 14 + your Proficiency Bonus, which means its baseline is 16, and it'll eventually go up to 20. It has HP equal to 5 + five times your Ranger level (and has d10 hit dice equal to your ranger level). So, 20 HP to start, scaling up eventually to 105.
And it has a 40 ft movement speed.
When you summon the drake, you choose one of the damage types of the Chromatic/Metallic dragons, which determines its damage immunity and the damage of its Infused Strikes trait (which we'll get to).
Its bite attack has a +3 plus PB to hit (so +5 when you first get it, eventually scaling to +9) and deals 1d6 + PB piercing damage (5.5 on average at level 3).
It also has a reaction to use Infused Strikes - this lets the drake add 1d6 of its draconic essence damage type to the damage done by an attack made by a creature within 30 feet of it. (It can presumably use this on itself as well).
So, first off, while it's probably not quite as beefy as the Ranger, it's got decent AC and decent HP to mess it up in melee. In addition to your attacks, it's effectively adding 2d6+PB damage each round. You can't max out its strength to make its bite more likely to land, but it's not going to far too much behind.
Now, back to subclass features:
At 7th level, you get Bond of Fang and Scale. The drake grows wings, gaining a flying speed equal to its walking speed (which is 40 feet). Also, the drake grows to medium size, and you can ride the drake as a mount if your size is medium or smaller, though the drake cannot fly when carrying you.
This also adds 1d6 to the drake's bite attack of the damage type chosen for the drake's Draconic Essence. So, each bite is now doing 2d6+PB, with some of it magical damage.
Finally, you also gain resistance to the damage type chosen for its Draconic Essence.
Letting a Ranger use their animal (not beast, but still kind of an animal) companion as a mount is very cool, and the boost in damage here starts to make its output pretty significant for just part of your suite of abilities. Also, while it's supposed to be the same drake, being able to adjust the damage type for when you face some foe that uses that type is pretty great, and letting the ranger get in on that is pretty nice.
At level 11, you get Drake's Breath. As an action, you can exhale a 30-foot cone of damaging breath, or you can have your drake exhale it. You choose acid, cold, fire, lightning, or poison damage (and this does not have to be the same as the drake's Draconic Essence,) and each creature in the cone has to make a Dexterity saving throw, taking 8d6 damage of that type on a failure or half on a success.
So, to be clear, this is a fireball's worth of damage, which is impressive even at this level. And the damage scales up to 10d6 when you hit level 15.
You can use this once for free per long rest, but you can also expend 3rd or higher level spell slots to do it again. A 3rd level slot for an 8d6 aoe attack is definitely a good bargain, so you'll probably be using this a lot. Also, your (now flying) drake will be in a great position to angle this to hit the maximum number of foes.
Finally, at 15th level, you get Perfected Bond:
The drake's bite attack now deals another 1d6 damage of the Draconic Essence type (so a total of 1d6+PB piercing and 2d6 elemental damage).
The drake also becomes large in size, and when you ride it, you are no longer prohibited from flying.
Finally, when either you or the drake takes damage while you're within 30 feet of it, you can use your reaction to give yourself or the drake resistance to that instance of damage. And you can do this a number of times equal to your proficiency bonus per long rest.
So!
I think this is pretty strong. Not only does it really hit the fantasy pretty effectively (and man do I now want to fly around on my drake shooting things in an aerial battle) but I think that the features here provide enough oomph to make the subclass worth it.
Perhaps most importantly, getting the drake back is cheap. Hell, it could drop in the middle of the fight and you don't even need to wait for the fight to be over to get the drake back into the thick of it. That means that the drake can absolutely make sacrificial plays to protect the party - maybe even barring a deadly monster from following you or even grappling a foe in the middle of a harmful AoE.
The subclass isn't doing anything terribly shocking, but that's fine - it hits the fantasy effectively and I think would make for a powerful character with a pet that could easily become a beloved party mascot. (The art for the subclass is also adorable, with a dragonborn ranger getting their face licked by their drake pet just like an affectionate dog who doesn't realize how big they are.)