The easiest, and probably most competitive way to use Lurrus is in shells that alreday played few to no 3+ CMC cards, where it's pure value as a free-roll. One-drop aggro with all the Pulse Tracker variants, Soul Sisters, and "Cheerios" style decks with Puresteel Paladin and Sram, Senior Edificer +0-1 CMC equipment come to mind.
These cards have the advantage of working synergistically with Lurrus with no other pieces required, and if you can fit a few of them into a cohesive decklist, the value should flow.
Lin Sivvi, Defiant Hero could be sweet with traditional value rebels, a bunch of the best changelings (hello repeated 1 mana Taurean Maulers) or using both abilities to repeatedly recycle Children of Korlis to abuse large life payments or self-damage. There's a lot of possibilities there!
Another sweet use of Zirda's effect I didn't mention at first is repeatedly activating {x} costs at a value of 3 for just 1 mana. Lavaclaw Reaches, Demonspine Whip, and Hex Parasite can abuse this nicely to get huge on the cheap. Hex Parasite in particular can remove counters for no mana at all with a Zirda out if you pay with life, though I'm not sure if there's a really good way to abuse that.
While not as competitive as previously mentioned win conditions, there are a number of draw-trigger creatures that can benefit from windfalls and looting generally, and supercharge by Rielle. Chasm Skulker, Lorescale Coatl, Kydele, Chosen of Kruphix, Psychosis Crawler, and The Locust God come to mind. Surly Badgersaur may be a legitimate option just for the ability to mass-produce treasure when you're doing your thing. Also, it is total overkill, but for strictly humorous purposes Thought Reflection both doubles your initial draw from looting and windfall effects, then doubles Rielle's draw trigger too. That's a lot of cards.
Zirda, the Dawnwaker represents everything that the Companion mechanic can be in casual multiplayer: a source of boundless new possibilities for deck construction and fun and interesting new gameplay dynamics without (usually) being oppressive.
The obvious comparison is Training Grounds, which is itself a defining card in the "whacky fun" category of casual decks. Training Grounds has the benefit of costing a third as much mana and being far less vulnerable to most removal. Along with Biomancer's Familiar, you could already have up to 8 copies of the effect at a very cheap rate. They also allow you to include any card you want in your deck, with none of the Companion restrictions.
Zirda's Companion ability of course also offers you a guarantee that you will have access to your build-around effect in every single game. Just make sure you have some protection or recursion for Zirda if you're relying heavily on the effect!
Zirda enables wild new deck ideas, and avoids similarity to existing versions by imposing a new restriction. Here are a few of the brews I've come up with so far:
There are a thousand other possibilities! Create an undead army with Sedris, the Traitor King, cheaply re-equip Sunforger, go infinite with Basalt Monolith, or supercharge spells with Nivix Guildmage. I hope this thread sparks some inspiration, because I haven't been this excited about new deckbuilding in a long time!
Slightly less potent, but a lot more original would be decks which actually build around Lurrus and its ability. The natural way to abuse it would be to repeatedly sacrifice the highest-value permanents you can find for 2CMC or less. Some options which can sacrifice themselves include Arcbound Ravager, Children of Korlis, Black Lotus, Sakura-Tribe Elder, Coretapper, Engineered Explosives, Gonti's Machinations, Hope of Ghirapur, Kami of False Hope, Lion's Eye Diamond, Martyr of Frost, Martyr of Sands, Mishra's Bauble, No Rest for the Wicked, Quest for the Gravelord, Seal of Cleansing, Seal of Primordium, Seal of Fire, Selfless Spirit, Spore Frog, Urza's Bauble, Vampire Hexmage, and Wayfarer's Bauble.
These cards have the advantage of working synergistically with Lurrus with no other pieces required, and if you can fit a few of them into a cohesive decklist, the value should flow.
The other major way to build around Lurrus specifically is to make a deck with sacrifice outlets and fodder, and build around that theme more completely. Some permanents that get great value out of being repeatedly sacrificed and replayed include Blisterpod, Brindle Shoat, Cauldron Familiar, Tukatongue Thallid, Doomed Traveler, Feral Prowler, Festering Mummy, Grim Initiate, Hunted Witness, Mogg War Marshal, Primal Druid, Veteran Explorer, Viridian Emissary, Baleful Strix, Burglar Rat, Dockside Extortionist, Ichor Wellspring, Oath of Nissa, Sarcomancy, Satyr Wayfinder, Servo Schematic, Stitcher's Supplier, Stoneforge Mystic, and Yarok's Fenlurker.
You'll want to combine the sacrifice fodder with sacrifice outlets such as Altar of Dementia, Carrion Feeder, Goblin Bombardment, Viscera Seer, Witch's Oven, Skullclamp, and Priest of Forgotten Gods. Add some sources of value from death triggers like Blood Artist, Cruel Celebrant, Zulaport Cutthroat, and Impact Tremors and you have a deck!
While Lurrus makes it tempting to load up on permanents, mass reanimation such as Return to the Ranks, Immortal Servitude, Wake the Dead, and Rally the Ancestors can be outstanding, as can spell-based token makers like Sylvan Offering.
Ambassador Laquatus is probably best used to self-mill in a graveyard deck to set up things like the new Boneyard Mycodrax.
Lin Sivvi, Defiant Hero could be sweet with traditional value rebels, a bunch of the best changelings (hello repeated 1 mana Taurean Maulers) or using both abilities to repeatedly recycle Children of Korlis to abuse large life payments or self-damage. There's a lot of possibilities there!
Another sweet use of Zirda's effect I didn't mention at first is repeatedly activating {x} costs at a value of 3 for just 1 mana. Lavaclaw Reaches, Demonspine Whip, and Hex Parasite can abuse this nicely to get huge on the cheap. Hex Parasite in particular can remove counters for no mana at all with a Zirda out if you pay with life, though I'm not sure if there's a really good way to abuse that.
I'm interested in building a flash deck around Slitherwisp and Cunning Nightbonder.
Flourishing Fox and Drannith Stinger slot immediately into any cycling deck in their colors.
Eerie Ultimatum and Ruinous Ultimatum are great at the top end, and not quite as cheesy as Emergent Ultimatum in my opinion.
Quartzwood Crusher alongside Kaheera, the Orhpanguard make me want to take another look at beast tribal.
Some of the efficient-bodied mutate cards look like they have value as pump with upside. Growing things like Vault Skirge is kind of fun, but I'm more interested in mutating Cold-Eyed Selkie, Cephalid Constable, and Dreamstealer with Gemrazer, Otrimi, the Ever-Playful, and Sawtusk Demolisher.
Glademuse seems like a fun build-around, possibly slotting in the Slitherwisp flash deck if the color requirements could be accommodated.
The obvious comparison is Training Grounds, which is itself a defining card in the "whacky fun" category of casual decks. Training Grounds has the benefit of costing a third as much mana and being far less vulnerable to most removal. Along with Biomancer's Familiar, you could already have up to 8 copies of the effect at a very cheap rate. They also allow you to include any card you want in your deck, with none of the Companion restrictions.
What Zirda brings to the table is a much broader effect: the reduction of ALL activated ability costs and not only creatures. Zirda enables powerful cost reduction on artifacts like Mimic Vat, Loreseeker's Stone, and any equip costs. Enchantment activations like Gerrard's Battle Cry or Profane Procession work, as do lands like Inventors' Fair, Sea Gate Wreckage, or Shivan Gorge. You can get even more creative by reducing activation costs in other zones. Try the graveyard with Drownyard Temple or Rot Farm Skeleton, or your hand with ninjas like Ink-Eyes, Servant of Oni, or the cycling mechanic in general. Achieve maximum weirdness by cheapening activations from exile with Torrent Elemental, or opponents' permanents like Xantcha, Sleeper Agent! While using these unique effects, you can of course include Training Grounds classics like Duskwatch Recruiter, Eldrazi Displacer, and Kenrith, the Returned King.
Zirda's Companion ability of course also offers you a guarantee that you will have access to your build-around effect in every single game. Just make sure you have some protection or recursion for Zirda if you're relying heavily on the effect!
Zirda enables wild new deck ideas, and avoids similarity to existing versions by imposing a new restriction. Here are a few of the brews I've come up with so far:
4 Pack Rat
1 Magus of the Will
3 Urborg Syphon-Mage
3 Xantcha, Sleeper Agent
2 Olivia Voldaren
1 Geth, Lord of the Vault
1 Arguel's Blood Fast
4 Mire in Misery
4 Rakdos Signet
3 Talisman of Indulgence
1 Planeswalker's Scorn
4 Syphon Mind
4 Subterranean Tremors
4 Dragonskull Summit
4 Lavaclaw Reaches
1 Castle Locthwain
4 Mountain
6 Swamp
2 Carrion Feeder
2 Viscera Seer
2 Dread Wanderer
4 Priest of Forgotten Gods
1 Teysa, Orzhov Scion
4 Twilight Drover
3 Chittering Witch
1 Erebos, Bleak-Hearted
1 Sadistic Hypnotist
2 Vampiric Rites
2 Luminarch Ascension
1 Shadows of the Past
2 Attrition
2 Jar of Eyeballs
4 Sram's Expertise
2 Decree of Juistice
4 Concealed Courtyard
4 Isolated Chapel
3 Castle Ardenvale
1 Memorial to Folly
3 Plains
5 Swamp
3 Herald of Anafenza
4 Joraga Treespeaker
1 Oviya Pashiri, Sage Lifecrafter
2 Rhys the Redeemed
3 Jade Mage
3 Selfless Spirit
2 Elder of Laurels
1 Jazal Goldmane
3 Dawn of Hope
3 Luminarch Ascension
1 Mastery of the Unseen
3 Shamanic Offering
3 Sylvan Offering
3 Decree of Justice
4 Stirring Wildwood
4 Sunpetal Grove
2 Castle Ardenvale
3 Gavony Township
3 Forest
3 Plains
4 Flourishing Fox
4 Vile Manifestation
1 Yarok's Fenlurker
1 Scholar of Athreos
1 Splendor Mare
4 Archfiend of Ifnir
1 Horror of the Broken Lands
3 Demon of Dark Schemes
1 Soul of Innistrad
2 Scion of Darkness
1 Void Beckoner
2 Astral Drift
2 Cast Out
1 Unearth
1 Dirge of Dread
2 Decree of Pain
2 The Cauldron of Eternity
4 Caves of Koilos
4 Concealed Courtyard
3 Barren Moor
4 Secluded Steppe
1 Memorial to Folly
1 Vault of the Archangel
1 Plains
5 Swamp
4 Brighthearth Banneret
4 Creeping Trailblazer
3 Leafkin Druid
4 Smokebraider
3 Soulbright Flamekin
4 Incandescent Soulstoke
3 Inner-Flame Igniter
2 Creeperhulk
4 Harmonize
2 Sudden Demise
3 Tempt With Vengeance
4 Game Trail
4 Raging Ravine
4 Sheltered Thicket
1 Flamekin Village
1 Kessig Wolf Run
5 Forest
4 Mountain
2 Aphetto Alchemist
1 Azure Mage
1 Passwall Adept
3 Vedalken Aethermage
1 Voidmage Prodigy
3 Arcane Artisan
3 Patron WIzard
3 Riptide DIrector
1 Soratami Savant
1 Azami, Lady of Scrolls
1 Arcanis the Omnipotent
1 Niv-Mizzet, the FIremind
1 Memnarch
2 Firestorm
1 Sorcerer's Wand
4 Springleaf Drum
2 Illusionist's Bracers
1 Turbulent Dreams
1 Aggravated Assault
1 Mind Over Matter
4 Spirebluff Canal
4 Shivan Reef
4 Sulfur Falls
4 Wandering Fumarole
1 Riptide Laboratory
6 Island
1 Mountain
There are a thousand other possibilities! Create an undead army with Sedris, the Traitor King, cheaply re-equip Sunforger, go infinite with Basalt Monolith, or supercharge spells with Nivix Guildmage. I hope this thread sparks some inspiration, because I haven't been this excited about new deckbuilding in a long time!