5e Features as Magical Weapons and Items - Fighter

Fighter

Heartseeker - A weapon that would rather kill than wound. It scores a critical on attack rolls of 19 & 20. (Improved Critical)

Ring of Swiftness - This ring is forged from a mercury alloy, and bestows greater speed and reaction time to its wearer. While worn on the right hand, it allows it's wearer to act more swiftly; they can take another action when they choose, which is resolved at the end of a round. This ability is lost until the ring is worn for one hour on the left hand. (Action Surge)

Amulet of the Still Wind - An ancient talisman bestowed with the Wind of Peace. Once per day during combat, when the wearer forgoes attacking and moving, they recover 1d12 hit points. (2nd Wind)

5e Features as Magical Weapons and Items - Cleric

Shield of the Sun - This shield stores sunlight and releases it an a bright flash when the bearer is in danger. When the shield is charged, the bearer can force a reroll on a successful attack made within 10 feet. (Warding Flare)

Chromatic Staff - A stave carved from the wood of the world tree; it is imbued with the chaos of uncreation. At dawn, it's bearer can meditate on one of the forces of nature, and attune it to heat, cold, or lightning. Instead of bludgeoning damage, the staff deals damage of the attuned type. (Divine Strike: Nature)

Lightning Chain - This chainmail crackles with static electricity. When struck by a metal weapon, the bearer can release the stored charge into the attacker, dealing 2d6 lightning damage. (Wrath of the Storm)

Mace of Sympathy - A mace made entirely of metal, yet incredibly light. Some legends claim that these maces were forged from the Caduceus. When casting healing spell while bearing the mace, the user is healed a number of hit points equal to the spell's level (Blessed Healer)

Relic of the Conqueror - A finger bone from Ihdon, Lord of War. After spending ten minutes in prayer holding the relic, you receive Ihdon's boon. For the next two hours, you can invoke the boon to add +10 to an attack roll of you or your allies. Each time you invoke Ihdon, it has half the chance of succeeding as the previous invocation. (War God's Blessing)

Gravedigger's Lantern - The light from this bullseye lantern is not for the living. Shining it on a creature draws it closer to death. The next attack that strikes the target deals twice the amount of damage. Once the lantern is opened and the light is shone, it loses this property until it is left for one hour under moonlight. (Eye's of the Grave)

Codex Benedictum - The blessed book of knowledge has literally countless pages. If you spend 10 minutes searching through the pages, there is a 3-in-6 chance of finding information on whatever topic you are currently trying to find. (Knowledge of the Ages)

5e Features as Magical Weapons and Items - Bard

I quickly burned out trying to post daily. I'm gonna aim for weekly.

Part two of this series:

Bard

[Instrument] of the Bard - Playing this instrument lifts the spirits of your allies. Once per combat, you can play a melody instead of attacking. Any ally who can hear the music gains d6 to their next attack roll. [Bardic Inspiration]

Cymbals of Distraction - When these finger cymbals collide, they produce the sound of full crash cymbals. Once per combat, when they are in your hand,  instead of attacking you can crash the symbols together to distract an enemy combatant as they attack an ally. When you do so, subtract d6 from their attack rolls. [Cutting Words]

War Drum - Beat terror into the hearts of your foes. When combat begins, you can begin playing the War Drum and forgo attacking. In doing this, each of your allies can roll a d6 and add to the damage of one of their attacks. Additionally, enemy Morale rolls are increased by your Charisma Modifier. [Combat Inspiration]

5e Features as Magical Weapons and Items - Barbarian

The lack of powers/features/feats available to characters can be quite a jarring difference between modern and old school play. To try and bridge that gap, it occurred to me to create magic weapons and items out of the many features that from 5e that can be used in B/X.

Barbarian

Axe of Bloodlust - Once per turn, an attack that doesn't hit may be re-rolled. In the round this is used, one missed attack against the wielder may also be re-rolled. [Reckless Attack]

Black Cap Mushroms - Eating one causes the character to enter into frenzy where pain isn't felt and their adrenaline is pumping. For one turn, attacks do +1 damage and damage received is reduced by 1. Eating more than one in a day causes a character to fall unconscious for 4d6 hours. [Rage]

Boots of Leaping - When activated, you have a flying speed equal to half your speed. This benefit works only in short bursts: you fall if you end a round in the air and nothing else is holding you aloft. Usable once per day. [Totemic Attunement: Eagle]

Sandstorm Hourglass - When you place this hourglass with the filled side up, a sandstorm engulfs the immediate area. In a 10 foot radius, boiling-hot sand whips around the glass, dealing d3 damage to anyone who enters or begins in that area.

Undying Amulet - When a character wearing the amulet is reduced to 0 HP, the amulet shatters, and the character is left with 1 hit point. [Rage Beyond Death]

Brutal Weapon - Critical hits made with a brutal weapon deal three dice of damage, instead of two. Costs ten times the amount of a regular weapon to purchase from a master smith.

Totem's - I'm too lazy to write them all out, but all the Aspects of the Beast make great magic items.

Next installment will feature Bard-y items