Abbey Matron
Abomination
Abu Ja'far
Abyssal Nightstalker
Adventurers' Guildhouse
Ærathi Berserker
Æther Searcher
Agent of Aquisitions
Aisling Leprechaun
Alaborn Grenadier
Alaborn Veteran
Alaborn Zealot
Aladdin's Lamp
Alert Shu Infantry
Ali Baba
Aliban's Tower
Ambush
An-Havva Constable
An-Havva Inn
An-Havva Township
Animus of Predation
Anti-Magic Aura
Apocalypse Chime
Apprentice Sorcerer
Arcane Savant
Archdemon of Paliano
Arenson's Aura
Armored Galleon
Army of Allah
Arrogant Vampire
Artifact Possession
Artifact Ward
Ashnod's Battle Gear
Assassin's Blade
Ayesha Tanaka
Aysen Abbey
Aysen Highway
Backdraft
Backfire
Baki's Curse
Ballot Broker
Balm of Restoration
Barbarian General
Barbarian Horde
Barbary Apes
Barbtooth Wurm
Bargain
Baton of Morale
Battering Ram
Bear Cub
Beast Walkers
Beasts of Bogardan
Besmirch
Bite of the Black Rose
Black Carriage
Black Mana Battery
Black Ward
Blazing Effigy
Blessed Wine
Blood of the Martyr
Bloodcurdling Scream
Blue Mana Battery
Blue Ward
Bog Rats
Border Guard
Borrowing the East Wind
Bottomless Vault
Brago's Representative
Brainwash
Brine Hag
Broken Dam
Brutal Nightstalker
Burning Cloak
Burning Fields
Burrowing
Caller of the Untamed
Camel
Camouflage
Canal Courier
Canal Dredger
Cao Ren, Wei Commander
Capital Punishment
Capricious Sorcerer
Castle Sengir
Cat Warriors
Cathedral of Serra
Cave People
Caverns of Despair
Celestial Prism
Chain Stasis
Champion's Victory
Chandler
Chaoslace
Charging Bandits
Chorus of Woe
Cleansing
Clergy of the Holy Nimbus
Cloak of Feathers
Cloud Pirates
Coastal Wizard
Cocoon
Cogwork Grinder
Cogwork Librarian
Cogwork Spy
Cogwork Tracker
Command of Unsummoning
Conch Horn
Conservator
Control of the Court
Coral Reef
Corrupt Court Official
Council Guardian
Council of Advisors
Counterintelligence
Coveted Peacock
Craven Knight
Creature Bond
Crevasse
Crown of the Ages
Crown-Hunter Hireling
Cruel Fate
Cruel Tutor
Cunning Advisor
Cunning Giant
Curse Artifact
Cursed Land
Custodi Peacekeeper
Custodi Soulcaller
Dakmor Bat
Dakmor Scorpion
Dakmor Sorceress
Damping Field
Dark Maze
Dark Offering
Dark Sphere
Daughter of Autumn
Deadfall
Deal Broker
Deathlace
Deception
Deep Water
Deep Wood
Deep-Sea Serpent
Defiant Stand
Deja Vu
Delif's Cone
Delif's Cube
Deputized Protester
Desert Drake
Desert Nomads
Desert Sandstorm
Devoted Hero
Devouring Deep
Djinn of the Lamp
Draconian Cylix
Drafna's Restoration
Dread Charge
Dream Coat
Drudge Spell
Durkwood Boars
Dwarven Armorer
Dwarven Hold
Dwarven Lieutenant
Dwarven Pony
Dwarven Sea Clan
Dwarven Song
Dwarven Trader
Dwarven Warriors
Dwarven Weaponsmith
Earthbind
Elder Spawn
Electric Eel
El-Hajjaj
Elven Fortress
Elvish Scout
Emerald Dragonfly
Empty City Ruse
Enchanted Being
Energy Tap
Enraged Revolutionary
Equinox
Erosion
Eternal Flame
Eternal Warrior
Eunuchs' Intrigues
Evaporate
Extinguish
Eye Spy
False Mourning
False Orders
False Peace
Fang of the Pack
Farmstead
Farrelite Priest
Fasting
Feast of the Unicorn
Feedback
Feint
Festival
Festival of Trokin
Final Strike
Fire and Brimstone
Fire Bowman
Fire Snake
Flamewright
Flanking Troops
Fleet-Footed Monk
Floral Spuzzem
Folk of An-Havva
Forest Bear
Forethought Amulet
Forget
Fortified Area
Foxfire
Frankenstein's Monster
Frozen Shade
Fruition
Fyndhorn Brownie
Game of Chaos
Garbage Fire
Ghost Hounds
Giant Albatross
Giant Shark
Giant Slug
Giant Turtle
Glyph of Delusion
Glyph of Destruction
Glyph of Doom
Glyph of Life
Glyph of Reincarnation
Goblin Artisans
Goblin Flotilla
Goblin Hero
Goblin Kites
Goblin Racketeer
Goblin Rock Sled
Goblin War Cry
Golden Bear
Golgothian Sylex
Gosta Dirk
Grave Robbers
Gravity Sphere
Gray Ogre
Great Defender
Great Wall
Greater Werewolf
Green Mana Battery
Green Ward
Grenzo's Cutthroat
Grenzo's Rebuttal
Grenzo's Ruffians
Grudge Keeper
Guardian Angel
Hand of Death
Harmony of Nature
Haunting Wind
Hazduhr the Abbot
Headless Horseman
Headstone
Heart Wolf
Heaven's Gate
Hell Swarm
Helm of Chatzuk
Heroism
Hidden Path
Highland Giant
Hipparion
Hollow Trees
Holy Armor
Homarid
Homarid Shaman
Homarid Warrior
Hornet Cobra
Howling Fury
Huang Zhong, Shu General
Hurr Jackal
Hyperion Blacksmith
Icatian Infantry
Icatian Moneychanger
Icatian Skirmishers
Icatian Store
Ichneumon Druid
Ignition Team
Illusion of Choice
Illusionary Informant
Imperial Edict
Implements of Sacrifice
Imposing Visage
Independent Troops
Indestructible Aura
Infernal Medusa
Infinite Authority
Infinite Hourglass
Ingenious Thief
Initiates of the Ebon Hand
Inquisition
Irini Sengir
Ironclaw Curse
Ironroot Treefolk
Island Fish Jasconius
Jandor's Ring
Jeering Homunculus
Jinx
Johan
Joven
Joven's Tools
Jovial Evil
Justice
Kasimir the Lone Wolf
Keen-Eyed Archers
Keepers of the Faith
King Suleiman
King's Assassin
Kiss of Death
Knights of the Black Rose
Kongming's Contraptions
Koskun Falls
Koskun Keep
Kry Shield
Lady Sun
Lady Zhurong, Warrior Queen
Lance
Land Leeches
Land's Edge
Leovold's Operative
Library of Lat-Nam
Lieutenants of the Guard
Life Matrix
Lifeblood
Lifelace
Lifetap
Living Armor
Living Artifact
Lizard Warrior
Lord Magnus
Lore Seeker
Lost Soul
Lu Su, Wu Advisor
Lurker

First up I can recommend cards that are unique mechanically. I don't know of any other cards that do quite what Backfire, Forget, or Drafna's Restoration do. Perhaps these aren't very good, but I've noticed something over the years. Unique effects randomly become good when the right card is printed years later. We all know the Cinderella tale of Illusions of Grandeur becoming an overnight star when Donate was printed. How much better does Forget look when (Hullbreacher) is in play? How good would Drafna's Restoration become if the next set had a card that revealed the top X cards of your library and put all artifacts onto the battlefield? King Suleiman hasn't been so hot over the last 25 years... but we can all imagine a metagame in which he's a bomb. Maybe one day!

Nettling Imp and Siren's Call might look similar to Alluring Siren and Grand Melee, but there's a huge difference: the clause about destroying creatures who aren't able to attack. This turns every Twiddle into Murder, every Icy Manipulator into Visara the Dreadful.

Related to that: cards that are unique in their color. Gravity Sphere may not be the only card to make all creatures lose flying (see Mystic Decree), but it's the only card in red. Chaosphere is not quite the same.

Phyrexian Gremlins is far from the best artifact removal in Magic, but it's near the top tier for mono black decks. In formats like commander that restrict your color options, this would absolutely see fringe play.

Revelation is worse than Telepathy... but it's green!

Sorceress Queen is identical to Serendib Sorcerer... but it's black! These things matter.

Potential commanders. You could argue Ayesha Tanaka is a little overcosted for its power level. I would argue that's the point. Commander players love a good challenge, and they love playing with obscure underpowered cards. (Even I will admit that Gosta Dirk may be too underpowered, though.)

Voting cards. We already have the voting mechanic, with Council's Judgment and Coercive Portal. Why not finish the thought, with cards like Grudge Keeper and Illusion of Choice? Illusion of Choice is a mini-Mindslaver for only one mana... it could lead to some of the rare instances of Carnage winning the vote!

Redundancy. I don't think we need Arenson's Aura when Teferi's Care exists (what deck would want 8?). But I can easily imagine a combo deck that would want 8 copies of Mobilize and Vitalize.

(Side note about Mobilize: I was stunned while cultivating the list of offline cards just how many have art by Rebecca Guay. There is definitely a demand for that alone, regardless of what the cards do. I mean look at these!!)

Wall cards. WotC decided at some point to stop caring about walls, even though they were a heavy focus in the first few sets. Well! That doesn't mean we want to stop playing with Ali Baba. There are niche formats where Tunnel is the perfect sideboard option (like my above '90s cube, for instance).

And that says nothing about the walls themselves! I wanted to make defensive red a viable option in my cube, but was severely limited due to the lack of Wall of Dust, Wall of Earth, and Wall of Heat. Why?

Three cards make a huge difference in a cube. Imagine how differently green ramp would play out in Vintage Cube without Rofellos, Llanowar Emissary, Gaea's Cradle, and Natural Order. (Also, I know it's not a wall, but can I just say how much I wanted Sisters of the Flame in my cube? Not currently an option, MORE'S THE PITY.)

Banding effects, like Baton of Morale and Mountain Stronghold: there are players who want these. Not many of us, but we're here.

Golgothian Sylex, well it might not be able to stop a Strip Mine or a Hurkyl's Recall, and it might be too slow to matter against Mishra's Workshop, but what about uh... Yawgmoth Demon or Shapeshifter?

OK we could probably do without Golgothian Sylex. It's not quite as relevant as City in a Bottle. But it least can destroy Mishra's Factory, so it's more relevant than Apocalypse Chime!

Multiplayer mechanics. We already have goad online, thanks to (Agitator Ant). But commander players aren't allowed to run 4x Agitator Ant. They need to supplement 1x Agitator Ant with Goblin Racketeer and Jeering Homunculus.

Joke cards. There is some novelty in reading the text boxes of Merchant Ship, Fasting, Truce, Winter's Sky, and Deep Water and marveling at how wildly unplayable they are. They're as funny as Wood Elemental but much less known. Wood Elemental was reprinted in ME4 solely for the lulz; let's do the same for these cards and expose them to a new audience!