Swap between weapons to inflict multiple mastery properties and gain Sen which you can spend on powerful attacks. Loosely baed off of FFXIV's Samurai job.

Level 3: Fuka

When you choose this subclass at level 3, you learn the ability to quickly swap weapons. Once per round, as a free action, you may choose a weapon or item in one of your hands. Store it away and equip another weapon in that hand. If the item takes two hands to pick up or wield, you may place the weapon or item in your other hand away.

Level 3: Iaijutsu Proficiency

Starting when you choose this subclass at level 3, when you inflict two different weapon mastery properties onto a creature; you gain a Sen.

You can hold one Sen at a time.

You also learn the Iaijutsu feature. Iaijutsu changes based on the number of Sen you have. Upon use, consumes all Sen.

1 Sen: Higanbana - Bonus action, make a melee weapon attack roll with your currently equipped weapon. It deals 1d4 damage.  

Level 7: Iaijutsu Mastery

Beginning at 7th level, you can store up to 3 Sen.

Iaijutsu 

1 Sen: Higanbana - Bonus action, make a melee weapon attack roll with your currently equipped weapon. It deals 1d4 damage. 

2 Sen: Tenka Goken - Bonus action, make a melee weapon attack roll with your currently equipped weapon to all creatures within 8 ft of you.

3 Sen: Setsugekka - Bonus action, the next attack you perform on a creature has advantage and deals an additional 1d6 damage times your proficiency bonus on one of the creatures hit.

Level 10: Prepared For Anything

Starting at 10th level, when you roll for initiative, you may replace one or both of the items in your hands with two others on your person.

Level 15: Hissatu

After reaching 15th level, when you equip a weapon, you may spend your reaction to attack with it.

Level 18: True Weapon Mastery

By the time you reach 18th level, you learn the mastery properties of all weapons.

Additionally, when you would inflict a weapon's mastery property, you may replace it with any mastery property effect.