Abdel Adrian, Gorion's Ward Acolyte's Reward Ancient Greenwarden Angelic Reward Artifact Ward Atraxi Warden Attentive Skywarden Auriok Bladewarden Bahamut, Warden of Light Baird, Steward of Argive Black Ward Bloodsworn Steward Blue Ward Boromir, Warden of the Tower Boseiju Reaches Skyward Broodwarden Cleric of the Forward Order Coward // Killer Coward // Killer Cowardice Crowd-Control Warden Cubwarden Curator's Ward Dawnhart Wardens Death Ward Denethor, Ruling Steward Duchess, Wayward Tavernkeep Edward Kenway Enduring Bondwarden Ephara's Warden Esior, Wardwing Familiar Essence Warden Fairgrounds Warden Faith's Reward Faramir, Steward of Gondor Fast Forward Final Reward Flickering Ward Gallows Warden Gandalf, Westward Voyager Gavi, Nest Warden Ghalma's Warden Ghost Warden Ghost-Lit Warder Goblin Coward Parade Goblin Wardriver Goldwarden's Helm Goldwardens' Gambit Green Ward Greenwarden of Murasa Grenzo, Dungeon Warden Guildscorn Ward Heart Warden Hixus, Prison Warden Homeward Path Honor's Reward Ifrit, Warden of Inferno Jadzi, Steward of Fate Jor Kadeen, First Goldwarden Kangee, Sky Warden Kavaron Skywarden Kin-Tree Warden Knight of the Skyward Eye Kopala, Warden of Waves Leonin Relic-Warder Liliana's Steward Littjara Glade-Warden Look Skyward and Despair Lukka, Wayward Bonder Lumengrid Warden Merry, Warden of Isengard Nemata, Primeval Warden

I don't have a strong counter to that, but I will say that 'walkers are plenty special already and that a spell that targets a planeswalker is, by definition, not just any old spell
This can be either through silver bordered cards that simply make the whole ordeal an absurd joke or by working around the limits of black bordered cards in original and imaginative ways
As we expect from MTG these days, most three-color creatures are legendary and thus Commander build-around cards
[1] Add 1 "Argostars" monster from your Deck to your hand, then if you can control a Continuous Trap in your monster zone OR control a Continuous Trap with an effect that Special Summons itself in your Spell/Trap Zone, you can immediately after this effect resolves, Normal Summon 1 Warrior monster