And hey, for the fun of it here are some racial subclasses! These will be themed after what makes each race "unique" (at least in my mind) from more of a genetic/ heritage standpoint, so that does mean that humans, gnomes and halflings (being as bog-standard as possible, or at least uninfluenced by primal desires/ outsiders, and are thus hard to come up with something thematic for) are out of the pool. Everything else, however, should have some kind of theme to work with...
Spoiler: Angelic guardian
Racial requirement: Aasimar, or Celestial creature type
When you choose this subclass at level 3 you gain the ability to actually heal your allies using the blood bond. As a bonus action you may roll 1 of your unspent hit die and add your Constitution bonus to the result, healing one of your blood bonded allies by the result shown.
Also at level 3 your body is reinforced by radiant energies, greatly expanding your reserves. The number of d12 hit dice you possess is increased by half of your proficiency bonus (rounded down).
Starting at level 7 your angelic nature lends power to your words. You gain proficiency in the Charisma (persuasion) skill, and you add half of your proficiency bonus to Charisma checks that you you are not proficient in.
In addition, you may roll 1d10 and add the result to your roll whenever you make a Charisma check before you know the results of this check. You must complete a long or short rest to regain use of this feature.
Potent healing bond
Starting at level 10 the amount you heal with your healing bond class feature is doubled.
Starting at level 15 you are able to regain a bit of strength when you benefit from your Crimson Revelry. When you benefit from your Crimson Revelry class feature, you may choose to regain up to 3 of your expended hit dice. You must complete a long rest to use this feature again.
At level 20 celestial energy pulses within you in an almost endless tide. When you heal a creature with your Healing bond class feature they regain the maximum amount of hit points possible. In addition, you become immune to radiant damage and necrotic necrotic damage and you no longer age by any means or have a maximum age.
Spoiler: Clan sworn
Racial requirement: Dwarf
At level 3 you are able to form a far more potent blood bond with one of your companions. After completing a long rest you may designate one ally with whom you have a blood bond as your clan bond. Whenever you use your reaction to reduce the damage being dealt to an ally who possesses your clan bond with your blood bond class feature, you may choose to absorb all of the damage instead of half, leaving them entirely unharmed. You may change who possesses your clan bond at the end of a long rest.
Also at level 3 you gain 1 additional maximum hit point for every level of Bloodsworn you possess.
At level 7 you resonate your blood with that of your terrestrial home. As long as you are within mountainous, underground or otherwise rocky terrain, your walking speed increases by 10 feet and you add double your proficiency bonus to all Wisdom (survival) checks even if you are not normally proficient in such checks. You always know which way is north, and the shortest path to the surface when you are underground.
At level 10 you may protect your clan bonded member almost completely from harm. You no longer need to use your reaction to absorb damage dealt to your clan bonded member using your blood bond, and can use your blood bond feature on your clan bonded ally any number of times during a given round.
Starting at level 15 you may designate up to two of your blood bonded allies as clan bonded allies at the end of a long rest.
At level 20 you become the ultimate guardian of your clan. When you use an ability that requires a long or short rest to recover, you do not expend it as long as you use it on an ally with a clan bond. Additionally at the end of a long rest your clan bonded allies gain 30 temporary hit points until their next long rest, and you may expend your hit dice to heal these allies during a short rest.
Racial requirement: Dragonborn or Half dragon
Starting at level 3 your blood bonds begin to awaken the draconic heritage buried deep within you. For each blood bond you possess your breath weapon increases the damage it deals by 2.
Also starting at level 3 your scales harden. You gain 1 additional maximum hit point for each Bloodsworn level you possess, and you increase your Armor Class by 1.
Starting at level 7 your senses sharpen immensely. You have darkvision out to 120 feet, advantage on Wisdom (perception) rolls that rely on hearing or smell, and you unerringly know the direction to any treasure trove within a mile of you (though you do not necessarily know which path to take to get there).
Starting at level 10 your breath weapon is faster to use and faster to recover. You may use your breath weapon as a bonus action, and if it is expended when you roll for initiative then you regain use of your breath weapon.
Also at level 10, whenever you use the Crimson Revelry class feature, creatures of your choice within 10 feet of you are dealt an amount of damage equal to your Constitution modifier. Damage dealt in this way is the same type as your breath weapon.
At level 15 your connection with your allies allows you to at least partially share the benefits of your heritage. Your blood bonded allies gain resistance to damage types that you are resistant or immune to as long as they retain their blood bond.
Blood dragon's form
At level 20 your body goes through an apotheosis, becoming more powerful and even more closely resembling the great beasts from which you are descended. You gain immunity to the damage type you are already resistant to from being a Dragonborn, and you gain resistance to any two of the following damage types: Acid, cold, fire, lightning or poison.
In addition you add your constitution bonus to damage rolls for your breath weapon, and enemies have disadvantage on their saving throws against its effect. You may choose to change your breath weapon's damage type to any damage type that a natural dragon (read, dragon without a template/ that is not a construct) possesses whenever you use it. This only changes the damage type for that use of the breath weapon, and does not affect your racial damage resistance.
Finally, you grow a pair of wings on your back. Typically these wings are made of blood, but they can also be made of flesh and bone at your discretion. Regardless of their form these wings give you a flying speed of 80 feet, and you can fuse them with your body or manifest them as a bonus action.
Spoiler: Exemplar of Humanity
Racial Requirement: Human or Half Elf.
At level 3, whenever you would expend one of your hit dice for any reason other than short rest healing, a willing blood bonded ally may give you one of their unspent hit dice for the roll. When an ally gives you a hit die in this manner, it is automatically expended.
Once an ally has given you a hit die in this manner, they must complete a long rest before they can do so again.
Strive for Greatness
Also at level 3, whenever you or a blood bonded ally make an attack roll, you may use your reaction to roll one of your unspent hit dice and add it to the attack. This feature must be used before you know the results of the original roll. Rolling a hit die in this manner expends it.
Jack of Many Trades
Starting at level 7 you gain proficiency in 2 skills and 2 languages of your choice.
Heart of Heroes
Starting at level 10, whenever an enemy hits a blood-bonded ally with an attack or that ally fails a saving throw, you may roll 1 of your unspent hit dice as a Reaction and add the result to either the ally's armor class or to their saving throw roll. This can change the result of the attack or saving throw. Rolling a hit die in this manner expends it.
Starting at level 15, whenever you use a feature that would cause you to expend one of your hit dice, you may roll a d6 instead of expending a hit die.
Champion of the Living
At level 20 you become the pinnacle of humanity's drive to protect and grow. Any class feature you have that targets one of your Blood Bonded allies no longer requires your Reaction to use and you gain expertise in all of your proficient skills.
Spoiler: Fey walker
Racial requirement: Elf, Half Elf, or Fey creature type
When you choose this path at level 3 your bonds to other creatures can be used to temporarily enter the feywilds, helping you move a bit faster. As a bonus action you may expend 1 of your unspent hit dice to teleport up to 60 feet, ending this movement within 5 feet of one creature with whom you have a blood bond. Moving in this way does not provoke an opportunity attack.
Also starting at level 3 you may expend 1 of your unspent hit dice to make a blood bond with an unwilling creature within 20 feet of you as a bonus action. This creature retains your blood bond for up to 1 minute, and this expends one of your maximum blood bond slots while it is in effect. You gain advantage on attack rolls against creatures with an unwilling blood bond.
At level 7 your fey blood allows you to emulate one of their more tricky abilities, the ability to alter your appearance. When you use this ability you can change your shape into that of one humanoid creature of your size or smaller you have a blood bond with, flawlessly emulating their appearance. You also have advantage on any check made to emulate their mannerisms and speech patterns, and if your target is still alive and conscious you can access their surface thoughts as an action. A creature can make a Wisdom saving throw (DC 8+Your proficiency bonus+your Charisma bonus) against this mental intrusion, and on a successful saving throw they prevent any further attempt for the next 24 hours.
You may remain in your chosen form for up to one hour, and your blood bond with the creature you are imitating may not be lost or broken during this time even if the creature dies. You may switch which blood bonded creature you are imitating as an action, but this does not reset this ability's duration.
You regain use of this ability after completing a long or short rest.
Starting at level 10 your fey ancestry warps the natural world ever so slightly. Enemy creatures treat all spaces within 15 feet of you as difficult terrain, and you always have at least half cover from ranged attacks.
Starting at level 15 you are able to use your blood bonds to manifest a concealing cloak of shadows around you. As a reaction when a creature damages you, you may expend 1 of your unspent hit dice to turn invisible and move up to your speed in a direction of your choice. You remain invisible until the end of your next turn. If you form a blood bond with a creature on your turn you extend this period of invisibility until you either finish a turn without forming a blood bond or you fall unconscious.
At level 20 you finally attain the pinnacle of your potential, and come to resemble the great lords and ladies among the Seelie and Unseelie courts. Your complexion pales or darkens, your eyes give off a bright glow that you can see by (up to 30 feet), and you gain an almost timeless look to yourself. Other aspects of your appearance can be changed in a manner fitting your character's personality. You no longer age in any way, you do not require any meditation or sleep to complete a long rest, and you can no longer be surprised.
You are also immune to exhaustion, the charmed condition and psychic damage. Your creature type changes to Fey if it was not already, and you are always treated as one size category larger or smaller than you are whenever it is advantageous to you. Finally, you may possess any number of blood bonds at any given time, and the duration of your Tricky Bond class feature is extended to 1 day.
Spoiler: Hellborn butcher
Racial requirement: Fiend creature type or Tiefling
Starting at level 3 your infernal taint begins to leak from you, coating your blade in its blazing, unholy glory. When you damage a creature with your melee weapon attacks you may choose to deal fire damage instead of your weapon's standard damage type.
Also starting at level 3 you gain the ability to mark creatures for execution with your blood bond. If you have an unused blood bond you may, as a bonus action, reach out and touch a creature to brand them. Whenever you deal fire or weapon damage to this creature you increase the damage dealt by your Constitution modifier. A brand lasts for 10 minutes. You may only have one brand active at a time
Starting at level 7 you begin muttering and speaking fell words that unsettle any creature near you. You have advantage on checks made to intimidate creatures, and creatures you choose within 15 feet of you have disadvantage on saving throws against the frightened condition.
Starting at level 10 when you brand a creature, they must make a Wisdom saving throw (DC 8 + your proficiency bonus + your constitution bonus) or become frightened of you and your blood bonded allies until the brand dissipates.
Starting at level 15 you and your blood bonded allies deal an additional 2d8 damage the first time they deal fire or weapon damage a creature with your brand each turn.
Master of Torments
Finally at level 20 you become a reaper of mortal life, marking and weakening any creature that comes in your reach. When you use your brand class feature you may mark any number of creatures in your reach. If you do so, you may not use your brand feature again until either all of the creatures marked by your brand are dead, or until 10 minutes pass. Additionally, any creature you have branded has disadvantage on attack rolls against your blood bonded allies and suffers disadvantage on Strength and Dexterity checks.
You no longer require an unspent Blood Bond to use your Brand class feature.
Racial requirement: Half Orc or Orc
When you take this subclass at level 3 you begin your journey to awaken your rage, becoming harder to frighten and influence. You have advantage on saving throws against the charmed and frightened conditions.
Also starting at level 3, when an enemy damages one of your blood bonded allies you have advantage on attack roll against that creature during your next turn.
Starting at level 7, when an enemy damages one of your blood bonded allies you add an extra damage die to the damage roll of all melee weapon attack you make against that enemy during your next turn. This damage die is the same size as your weapon's normal damage die (or one of the dice, in the case of a weapon such as the Greatsword.)
At level 10 your ferocious savagery hangs about you like a mantle. You add your Constitution bonus to your Strength or Charisma (intimidation) checks, and can make an intimidation check in combat whenever you kill your first enemy each turn.
Starting at level 15 your ancestral rage makes you more difficult to finish off. When an attack would reduce you to 0 hit points you may activate fury to be dropped to 1 hit point instead. As long as you remain at 1 hit point enemies have disadvantage on attack rolls against you, you do not take any damage when you use your blood bond to protect an ally, and you may reroll all of your weapon's damage dice, taking the new result.
While fury is active you make a Constitution saving throw whenever you are damaged with a DC of 15 or 1/2 the damage dealt to you (whichever is higher). If you succeed on this saving throw, you ignore all of the damage dealt and remain at 1 hit point. After 1 minute, if you are not healed, you fall unconscious but are stable.
You must complete a long rest to regain use of this ability.
At level 20 your vengeance and that of your allies is eternal. When an enemy damages one of your blood bonded allies you have advantage on all attack rolls against that creature until that creature is dead. Additionally, your allies will have advantage on their next attack against that enemy, and the next attack that enemy makes will be made with disadvantage.
(Fluff to come later, just getting the crunch out now)