Never Slaked, Always Seeking (The 5e Fighter)

"Not every member of the city watch, the village militia, or the queen’s army is a fighter. Most of these troops are relatively untrained soldiers with only the most basic combat knowledge. Veteran soldiers, military officers, trained bodyguards, dedicated knights, and similar figures are fighters."

Anyone who loves the Fight is a Fighter. Others know the Fight, or think they do, acknowledge it begrudgingly, but they do not love it like you do. The Fight does not hang on their shoulders, does not follow them to bed and kiss them gently awake in the morning.

(And the Fight does not love them either, it couldn't, not as much as it loves you. It knows you get so jealous, after all.)

They don't get class levels. You do.

One Easy Trick to Make Your Fighter Last All Night!

At level 1, you can spend Hit Dice as a bonus action without short rest-ing; roll to heal result+CON.

Other classes run out of Hit Dice. Fighters do not. Fighters always have more hitting to do. Once per day, you can use your Second Wind to get all your Hit Dice back.

[Digression 1: The fighter resource loop is a straight line. Fighters go all day, no peaks, no valleys. It's what separates them from 10-ish casters and half-casters. Barbarians run out of Rage. Monks run out of Ki. Fighters keep on trucking.]

At level 3, you get Combat Superiority, EXCEPT you spend your Hit Dice instead of superiority dice for Maneuvers; d10s, or a mix if you multiclass. You start with 2 of the following Maneuvers, in addition to the spend-HD-for-HP move from level 1:

• Disarm. When you hit, spend any number of HD to force the enemy to drop an object held with that many hands.
• Execute: When you hit, spend any number of HD to deal [sum] bonus damage.
• Feint. As a bonus action, spend 1 HD to gain advantage on your next attack against a nearby enemy.
  
• Lunge. As a bonus action, spend 1 HD to increase your reach by 5 feet.
• Parry. As a reaction, spend any number of HD to reduce incoming damage by [sum].
• Protect. As a reaction, spend any number of HD to reduce incoming damage for an ally within reach by [sum].
• Riposte. As a reaction, spend 1 HD to attack back when an enemy misses you with an attack.
• Shove. When you hit, spend any number of HD to push the enemy 15 feet away from you if they are that many sizes larger than you.
  
• Sweep. When you hit, spend any number of HD to deal [sum] damage to that many other enemies next to the target.
  
• Trip. When you hit, spend any number of HD to knock the enemy prone if they are less than that many sizes larger than you.

[Digression 2: If I could strip Hit Dice out of 5e and replace it with my feasting rules, I would. That said, they're much more fun if you can spend them shoryuken-ing the big bad.]

You can learn more Maneuvers from wandering martial artists, legendary swordsmen, etc. by training with them for 1 session. (Play a different character during this session.)

You also choose a School of Battle. This replaces your Martial Archetype, gives you additional features at 7, 10, 15, and 18, and expands your selection of Maneuvers.

Your School of Battle gives you an Impetus--your love language for combat. You regain Hit Dice by accomplishing the task specified by your Impetus. You'll see what I mean in a second.

Also, you get Know Your Enemy at or before level 3. Level 1 is probably fine.

School of Battle: Champion

The Fight is a competition, and you are nothing if not a competitor. Peerless on the track and in the arena, you turned your gaze above what mortal contests have to offer. Others see your potential, see what you can become; you are eager to live up to their expectations.

Impetus: Excellence. When you crit on an attack, refresh 2 HD.

[3] You crit on attacks twice as often. 

[7] You jump twice as far, swim twice as fast, and hold your breath twice as long.

[10] You choose an additional Fighting Style.

[15] You crit on attacks thrice as often.

[18] Once, instead of dying in combat, you may become a demigod with 100 HP. (Ignore age and disease to your liking. A minor cult worships you now.)

Champion Maneuvers:

• Challenge. When you hit, spend 1 HD to give the enemy disadvantage when attacking anyone but you until the end of your next turn.
• Decimate. When you defeat an enemy, spend 1 HD to attack again.
• Excel. As a reaction, spend 2 HD to pass a saving throw.
• Immortalize. When you defeat a significant enemy, spend 1 HD to immortalize whichever weapon (or unarmed appendage) dealt the killing blow. It's a magic weapon with a minor enchantment of your DM's choosing (perhaps rolled on their table of choice).

School of Battle: Eldritch Knight

The Fight is an ongoing experiment, and you are a tireless researcher. The greatest magicks are fueled by sacrifice; thus, you reason, proximity to violence will catalyze your sorcerous discoveries. You are certainly not mad; this is simply the scientific method at work.

Impetus: Hubris. When you cast a non-cantrip spell, refresh 1 HD.

[3] You ignore all class, race, and level requirements on the use of magic items.

[7] When you cast a spell, you may make a weapon attack as a bonus action.

[10] When you hit with a weapon attack, the target has disadvantage on saving throws against spells you cast before the end of your next turn.

[15] When you use your action surge or second wind, you may teleport up to 30 feet.

[18] When you cast a spell, you may cast it again as a bonus action.

Eldritch Knight Maneuvers:

• Confiscate. When you hit, spend any number of HD to Concentrate on a random spell of that level your target knows. Until you lose Concentration, they cannot cast that spell and you can cast that spell once.
• Ken. As a bonus action, spend any number of HD to Concentrate on a random spell of that level you have previously cast. Until you lose Concentration, you can cast that spell once.
• Plagiarize. As a reaction, spend 1 HD to Concentrate on a spell as someone else casts it. Until you lose Concentration, you can cast that spell once.
• Pry. As a bonus action, spend 1 HD to see every spell known by anyone who looks you in the eyes.

School of Battle: Martial Artist

The Fight is universal truth, and you are a devoted ascetic. You see yourself as you are; an especially sharp stone cutting through a world designed for combat. Your commandment is singular: Reach Heaven Through Violence.

Impetus: Balance. When you lose Concentration, refresh 1 HD.

[3] Your unarmed damage die is d4. Add your proficiency modifier to Concentration saving throws.

[7] Your unarmed damage die is d6. Your hands and feet are magic weapons.

[10] Your unarmed damage die is d8. You can Concentrate on 2 things at once.

[15] Your unarmed damage die is d10. Your hands and feet are invulnerable and indestructible.

[18] Your unarmed damage die is d12. You can Concentrate on everything, everywhere, all at once.

Martial Artist Maneuvers:

• One Mind Parts the Veil. As a bonus action, spend 1 HD to Concentrate. As long as you Concentrate, you may pass a Wisdom, Intelligence, or Charisma saving throw as a reaction.
• Twelve Eyes Find the Path. As a bonus action, spend 1/2/3 HD to Concentrate. As long as you Concentrate, you provoke no opportunity attacks. If you spent 2 HD, you can walk on walls. If you spent 3 HD, you can walk on water.
• Hundred Hands Turn the Rain. As a bonus action, spend 2 HD to Concentrate. As long as you Concentrate, you may deflect any projectile within reach as a reaction.
• Thousand Fists Break the Earth: As a bonus action, spend any number of HD to Concentrate. As long as you Concentrate, you may make that many unarmed attacks as an action.
  

School of Battle: Warlord

The Fight is your family, and you are a doting parent. You couldn't have been more proud watching your kids decimate their first hobgoblin battalion. It warms your heart when everyone gets together for a group interrogation. That's family: can't live with 'em, can't kill without 'em.

Impetus: Leadership. Once per turn, when an ally attacks on your turn, refresh 1 HD.

[3] Your allies have +20 movement speed on your turn.

[7] You may take an additional reaction between your turns.

[10] As a bonus action, you can shout at an ally to remove the frightened, charmed, or dazed conditions.

[15] Your allies have resistance to all damage types on your turn.

[18] If an ally would die, you may sacrifice yourself to restore them to 100 HP instead.

Warlord Maneuvers:

• Aim For The Eyes! When you hit, spend 1 HD. The next attack against the target by someone other than you has advantage.
• Get Over There! When you hit, spend 1 HD and an ally's reaction to let them move without provoking opportunity attacks from the target.
• Strike Them Down! When you attack, spend 1 HD and an ally's reaction to let them attack instead.
• Take Heart! As a bonus action, spend any number of HD to give that many allies temp HP equal to [sum] + your Charisma modifier.

School of Battle: Wrangler

Impetus: Arrogance. At the end of your turn, if you are wrangling (i.e. grappling, climbing, or riding) a creature larger than yourself, refresh 1 HD.

[3] You have advantage on attacks if you have the high ground.

[7] When you wrangle an unwilling creature, you choose whether they contest you with Athletics or Acrobatics.

[10] If you wrangle an unwilling, non-sentient creature for 3 consecutive turns, you control it entirely until you dismount.

[15] You can't be hurt by anything you are wrangling.

[18] You have all special abilities of anything you are wrangling.

• Break Their Spirit. When you hit, spend 1 HD to make the target frightened of you until the end of your next turn.
• From The Skies. When you take damage from falling, if you landed on a creature, spend 2 HD to deal that much damage to them instead.
• Hit The Deck. As a reaction, spend 2 HD to pass a Dexterity saving throw and/or move up to 10 ft.
  
• Yip Yip! As a bonus action, spend 1/2/3 HD and the reaction of a creature you are wrangling to move both of you 10/20/30 feet.

School of Battle: Magical Girl

Impetus: Applause. When you spend your entire turn posing, shouting, and/or transforming, refresh 2 HD.

[3] You soul-bond with a weapon. It's a magic weapon with a minor enchantment of your DM's choosing (or rolling). You can transform any weapon of the same type into this weapon; they transform back if they leave your possession.

[7] As an action, you may begin (or continue) transforming. While transforming, you can't move or attack, you are immovable, invulnerable, and indestructible, and you can stack temporary hit points from multiple sources. At the start of your next turn, triple your temporary hit points.

[10] You have a 30 ft flying speed while you have temporary HP.

[15] You may spend actions, bonus actions, and movement you didn't use during your last turn.

[18] Once, by gathering the simultaneous attention and devoted worship of 125.8 million sentient creatures, you may deal 9 billion damage to everything within a 1-mile radius of a point you can see.

Magical Girl Maneuvers:

• Beautiful Butterfly Beam! When you hit, spend any number of HD to deal the same damage to that many additional targets in a line in front of and behind your target.
• Glittering Gamma Ray Guard! As a reaction, spend any number of HD to gain [sum] temporary HP and resistance to your choice of damage type this turn.
• Pretty Penguin Power Up! As an action, spend 3 HD to deal double damage during your next turn.
  
• Shining Scepter Strike! When you hit, spend any number of HD to deal [sum] + your Charisma modifier more damage.
  

Secret Techniques

These maneuvers aren't core to any Schools of Battle, and can only be learned from eccentric martial artists with drinking problems:

• Fletcher's Gambit. When you attack from range, spend 2 HD to attack invisible targets, targets behind walls, etc. with no penalty. The projectile will even curve around corners.
  
• Like a Ghost. As a bonus action, spend 5 HD to become invisible. If a Void Monk sees you do this, they'll put a hit out on you.
• Roots of the World Tree. As a bonus action, spend 1 HD and plant your feet to become absolutely immovable.
• Dark Sky Ritual. As an action, spend any number of HD to make a single ranged attack against twice that many targets. Roll damage separately.
• Vigil. Spend 1 HD to make an opportunity attack without spending a reaction.
• Iron Cloth. Spend 1 HD to treat cloth as steel. OR Spend 8 HD to treat steel as cloth.
• Exploding Point Excavating Burst. Spend 6 HD to cast Fireball centered on the tip of your finger.
• Piledriver. Spend any number of HD to deal [sum] damage to a grappled creature.
• Santoryu. Spend 3+ HD to wield that many swords without penalty.
• Half-Sword. Spend 1 HD to deal bludgeoning, slashing, or piercing damage instead of another type.
  
• Shadow Boxing. Spend 1 HD to treat a creature's shadow as that creature.
• Kame... Hame... Spend any number of HD to Concentrate. At the start of your next turn, if you didn't lose Concentration, deal that much damage in a line.
• The World. Spend 12 HD to take another turn.
• Ki Rata. Spend any number of HD to multiply the damage of your next unarmed attack by that much.