This set absolutely rules, and I think I'm especially drawn to it because it's a straight up zero-irony magical-fantasy collection of cards, and a real return to form for Magic worldbuilding. We're here to explore the land of Tarkir, interact with its peoples and cultures, and learn new magic spells to add to our libraries. Remember? That's what the game is actually about.
Where's Loot? What's Kellan up to!? Who gives a FUCK.
Anyway, I'm adding a lot of cards to
Mutlivalent for this one. Maybe too many! So this is going to be a long post!

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Mage's Attendant
This Ox seems insane to me. Pure fuel for all aristocrats and token strategies. MV3 for all the "little reanimator" spells and effects. High toughness and vigilance makes it a great carrier for enchantments and equipment. My dream play: T3 Packbeasts, T4, attack and cast a kicked
Warhost's Frenzy. I mean, holy shit.

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Timely Reinforcements
This is just the kind of card I like in my instant/sorceries section. A great menu of effects that support multiple decks in different ways, plus a fun in-game puzzle to try and get the mana discount. This switch also puts more agency in the hands of the caster rather than their opponent.

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Countervailing Winds
Countervailing Winds has been in the cube for a while, kind of coasting on its cycling ability and loose synergy with the Sultai graveyard value decks. I don't find it to be a thrilling card, and I'm happy to make this switch to a more flexible card that still has a build-around feature but will work fine in any blue deck.

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Tragic Slip
Tragic Slip is an old favorite of mine, but Worthy Cost is so similar and the art/flavor is SO cool that I'm going ahead and switching.

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Escape Velocity
Another case of very similar mechanical effects that I'm switching for art/flavor reasons. Wild Ride's larger power buff is also much more exciting to me, even if it's only temporary. I mean, I'd LOVE to hit a
Khenra Spellspear or
Tenth District Legionnaire with this.

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Invasion of Zendikar
This is a total score for my newly-minted WUBRG deck. My main wish for that deck was a really strong 5c creature at uncommon, and the Reliquary Dragon token serves that purpose quite well, while the relic itself provides fixing and ramp for the rest of the deck. Basically an ideal card for the archetype. The loss of Invasion of Zendikar is probably the death-knell for Battles in my cube.

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Jewel Thief
Jewel Thief is a cool stats monster whose treasure token is always nice in a multicolor-heavy environment, but it's such an obviously good card that it just doesn't feel very unique. Meanwhile Sage of the Fang supports the light +1/+1 counters deck I'm working into the environment, plays nicely with the self-mill graveyard stuff, and has very cool art.

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Savai Crystal
This is going to be a full-cycle switch. The monuments don't ramp, but they cost 1 mana less. They don't cycle, but they have cool token-generating effects. The crystals have been in this cube for over 10,000 years, so I'm honestly just looking for a change. We'll see about these.

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Vraska, Swarm's Eminence
I have been flirting with a Golgari counters deck in this cube forever, and I have switched Vraska out for
Corpsejack Menace every time I theorycraft the archetype. Host of the Hereafter is just too beautiful to ignore. What an exciting card to see in a pack! Incidentally, a lot of new +1/+1 counter cards have recently arrived in the cube for other archetypes, so the time is ripe. This switch has three sub-switches:
Hagra Constrictor is in for
Undercity Scrounger,
Skyclave Shadowcat is in for
Stir the Sands, and
Untamed Kavu is in for
Silhana Ledgewalker. We'll see if this is fun to play. I hope so!
OK, now it's time for the big show. I'm going all-in on the Omen dragons available at Peasant rarities. I love them, and I want them, and I think by running all of them it'll make it easier to handle them alongside Adventures. I really wish the alternate-frame versions of these cards included reminder text for the Omen subtype, because I think that would be a useful visual cue. In any case, this is a big spend on my complexity budget and another nail in the coffin for Battles (currently the most complex things in the cube by a wide margin).

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Warchanter Skald
Grizzly Bears are always solid in draft, so this card has a good floor. Otherwise, this is the least exciting of the Omen dragons for me. Warchanter Skald is out because I think you have to work a little too hard for his payoff.

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Sea Hag
I think this is a pretty clear power downgrade, but I like the way Dirgur Island Dragon plays with
Icewrought Sentry and Prowess attackers like
Seeker of the Way. Plus, cutting one of my Adventure cards brings the Omens closer to parity.

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Goremand
Bonus card for the new Golgari counters deck, plus some synergy with Heroic. I'll miss Goremand a lot though, so this is the switch I'm most likely to undo.

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Invasion of Mercadia
Easy switch. Battle stonks in shambles. Sell now!

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Deadwood Treefolk
I like this guy with Domain, and I like that he's a bit more aggressive than the Treefolk. Plus, fewer counters to track. This switch has a subswitch:
Slavering Branchsnapper is in for
Nurturing Bristleback. This is basically for the sake of my green mana curve.

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Skyfisher Spider
I like Skyfisher Spider, but not as much as this dragon.

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Start // Finish
This is the weirdest card in the cycle, in my opinion. Seems very strong, though.

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Mercurial Geists
I had to have this guy. He's perfect! Mercurial Geists have been in the cube a long time even though I never found them very exciting. They're here for the spellslinger archetype, and I have no other attachment to them.

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Rip Apart
Rip Apart has been on borrowed time for a while, and this very Boros dragon is taking its place.

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Wavesifter
I have no interesting thoughts about this switch, but I want to point out that "Dynamic Soar" is word salad, and makes zero attempt to describe the effect of the spell. It ought to be called "Big Yell" or something. "Groan of Engorgement" maybe.