Dragon Rider

The Dragon Rider is an subclass of the Fighter class. Fighting side by side with their draconic companion, they are a force to be reckoned with. Some fighters are granted a blessing from time to time, they have, in some way or another, earned the trust of dragons, who in turn, have shown this trust by connecting them to a draco. Dracos are beings created by dragons through magic. They are around the size of dragon wyrmlings, and they don't grow much bigger, in the wild they are also terribly weaker than their creators. These creatures were created to serve the dragons. But when a dragon grants a fighter a draco, their loyalty shifts completely to their new master. And with this connection, and great training, they achieve greater powers than they would ever get from a life of nothing but servitude. Together, a rider and their draco are unstoppable.

Bonded Draco

Starting at level 3 you bond with a draco. To form this bond you take 8 hours, and must offer 50gp to whoever granted your companion. If your companion is ever slain, you can use the bond to restore it to life. With a 8 hour ritual and an offering of 25gp, you call forth your companion's spirit and bring it back to life, even if you don't have a single part of it's original body.

Your companion gains a variety of benefits while bonded to you. The draco gains an elemental breath attack that it can use once per short or long rest. The breath attack's damage starts at 2d6, and gains +1d6 every 3 levels of fighter. Its DC is 8 + your Intelligence + your proficiency bonus, and its range starts at 15 feet for cone breaths and 5 by 30 feet for line breaths, and at levels 7, 10, 15 and 18 they gain 5 extra feet to their range. The type of DC and the shape of the breath are based on the type of dragon they come from.

The companion obeys your commands as best it can. It rolls for initiative like any other creature, but you determine its actions, decisions, attitudes, and so on. If you are incapacitated or absent, your companion acts on its own.

Your companion has abilities and game statistics determined in part by your level. Your companion uses your proficiency bonus rather than its own. In addition to the areas where it normally uses its proficiency bonus, a draco companion also adds its proficiency bonus to its AC and to its damage rolls (the damage it gains from your proficiency bonus has the same type as its breath weapon). Your companion gains proficiency in two skills of your choice. It also becomes proficient with 4 saving throws of your choice. For each level you gain after 3rd, your draco companion gains an additional hit die and increases its hit points accordingly. Whenever you gain the Ability Score Improvement class feature, your companion’s abilities also improve. Your companion can increase one ability score of your choice by 2, or it can increase two ability scores of your choice by 1. As normal, your companion can’t increase an ability score above 20 using this feature unless its description specifies otherwise.

You also gain the benefit of learning to speak, read and write draconic.

Mounted Combat

At 7th level, your draco is even more helpful in combat while you're mounting it. While you are mounted, your melee weapons gain 5 extra feet to their normal reach, thanks to the draco's limber body, that helps you move in the field of battle. Also, if you make a jump from the back of your draco, you gain a bonus equal to the draco's strength modifier in feet for the distance of your jump.

You can menace your foes, foiling their attacks and punishing them for harming others. When you hit a creature with a melee weapon attack, you can mark the creature until the end of your next turn. This effect ends early if you are incapacitated or you die, or if someone else marks the creature.

While it is within 5 feet of you, a creature marked by you has disadvantage on any attack roll that doesn’t target you.

In addition, if a creature marked by you deals damage to anyone other than you, you can make a special melee weapon attack against the marked creature as a bonus action on your next turn. You have advantage on the attack roll, and if it hits, the attack’s weapon deals extra damage to the target equal to half your fighter level.

Regardless of the number of creatures you mark, you can make this special attack a number of times equal to your Strength modifier (minimum of once), and you regain all expended uses of it when you finish a long rest.

Linked Breath

At level 10, you can share your companion's breath. You can use an action to make a breath attack, that works exactly like your draco's breath. This feature expends the draco's use of their breath until their next short rest.

At 10th level, you become a master of locking down your enemies. Creatures provoke an opportunity attack from you when they move 5 feet or more while within your reach, and if you hit a creature with an opportunity attack, the target’s speed is reduced to 0 until the end of the current turn.

Greater Draco

At 15th level, your draco has grown stronger than any wild draco, thanks to all of it's training, and all of your care for it. It's Constitution and Strength both grow by 2 (this increase can go above the usual 20 limit) Its breath weapon's range grows by 15 feet and its walking and flying speeds both increase by 30 feet respectively. It also gains resistance to one type of damage of your choice between, bludgeoning, slashing or piercing. 

Starting at 15th level, you can run down your foes, whether you’re mounted or not. If you move at least 10 feet in a straight line right before attacking a creature and you hit it with the attack, that target must succeed on a Strength saving throw (DC 8 + your proficiency bonus + your Strength modifier) or be knocked prone. You can use this feature only once on each of your turns.

Draconic Link

At 18th level, your bond to your draco has grown so much, its granted you, part of a dragon's heritage. You and your draco can communicate telepathically (in draconic) with each other from any distance, as long as you are in the same plane. You also gain scales, that protect you, granting you a bonus of 1 to your AC, and resistance to your draco's type. You also gain other draconic features, like horns, claws and maybe even a tail, but these are purely cosmetic (unless your DM allows for anything).

 Starting at 18th level, you respond to danger with extraordinary vigilance. In combat, you get a special reaction that you can take once on every creature’s turn, except your turn. You can use this special reaction only to make an opportunity attack, and you can’t use it on the same turn that you take your normal reaction.