After having played nearly 2000 battles in the new Modern, I wanted to update my tier lists. Here are my new tiers for each rarity of card in Conclave Arcana and some notes on the big movers.
LEGENDARY
Graybrow moves to #1: While Gramel is great (he was my previous #1) and is actually rather versatile for an 11 mana card, AND will get better as Shapeshift sees more value with future cards, he simply isn't as useful as the other 3 cards that I put above him. Graybrow, namely, is the best card in the set if you ask me. There are some clear counters to Graybrow, but his utility in tons of different rulesets and mana caps and his synergy with various key summoners makes him the #1 legendary card.
Inept Healer and Jicarilla pass Gramel: These two superpowered healers overtake Gramel as well. Inept Healer's Corrupted Healing (with a strong kit to boot) and Jicarilla's Echo/Tank Heal/Rearguard cleanse plus her awesome pairing with EMP are just too powerful in the current meta.
Bellowmancer is S Tier: He's not just CA's best tank, he's one of the game's best (and most versatile) tanks.
Lots of downgrades: I was overly generous in my initial ratings of the Legendary class. Bren's great, but more of an A tier as the WT doesn't frequently play. Hemlig is powerful but situational, as is Palifala. Craniax is really hard to use (perhaps A tier in Wild). Zargrul and Desolation just don't get enough usage to be above C tier.
EPIC
Sentinel's S tier: Given it's one of only 2 units in the new Modern that can revive others at the moment, and its pairing with Heloise as well as Bloodline Tribute units, it's deserving of an S Tier classification.
Warwall Juggernaut's too slow and expensive: The kit looks great (besides the oddly placed Amplify), but there's a reason it never makes it into Hippo lineups. It's too expensive and too slow compared to alternatives.
RARE
Arcane Skinwalker is S Tier:@bjangles was right, this guy's the top Rare card in CA. Super versatile, and Silence makes him invaluable with Kelan being so powerful in today's meta. And Shapeshift will keep making him stronger over time.
Water Diviner goes up a notch: Still A tier, but higher up than before. This little guy, paired with Razi especially but also in B2B and low mana matches, is absolutely crucial.
Sheik goes up too: The Life Leech + Painforge mix on a magic card is awesome, especially with Lorkus's magic buff but in other situations as well. He can be on a Cryptic lineup with Water Diviner to trigger the Painforge. Very versatile, and great value at 6 mana.
Forked Tongue Sage super versatile: For 2 mana this guy gets used all the time, and he has decent attack and is a decent meat shield.
Sacrosant Meat Shield: The revive + divine shield alone gets this to mid/high B tier.
B level downgrades: Several cards just don't get used as much due to alternatives and the current meta, so Anyu, the Shieldmaiden, and Flame For Hire all went down within the B ranks.
C Tier shuffles: Thragha's kit looks better than her actual utility, probably due to that speed. Swiftbeard Stout's good as a taunt, but the rest of his kit is lackluster and the Bloodline Tribute on a ranged-only unit is mediocre. Archer Koi's too slow and its Fury/Painforge combo is a waaste - you get him only for the Repair which is a shame. Meanwhile Canary Keeper moved up slightly in my C-Tier ranks due to its damage output and speed at 2 mana.
COMMON
Cavewise Finds His Way to S Tier: The Cavewise Wayfinder is one of my favorite cards in the set. He gets less love than he should likely due to Graybrow, but often he can even outdo the brow. With the Giraffe behind him he's a beast, and with Lorkus giving him LL he can quickly rack up unreasonable amounts of health. Definitely S Tier.
Best Offensive Hippo: The Glamorous and Thunderous Herald is the best offensive Hippo in the game. Better than the Mudbinder (though they're both A Tier).
Excavator and Venomancer fall in the A Tier: They're both good cards, but a bit too situational. Venomancer gets a downgrade due to the Giraffe's existence, as that often wins out over her in Drath lineups, but I feel the Venomancer's being under-utilized in the current meta. As for the Excavator, he's mainly a Painforge igniter, though he has standalone value due to the damage-to-cost ratio. Both still A tier.
Lots of movement in the B tier: I won't go over each of the upgrades, but there are various cards I moved up in the B tier, or moved up from C to B. These commons are each quite usable, with the first 3 (Silverleaf Warlock, Mad Kaboomist, and Bludgeon Huckler) seeing the most use. The Brigand makes his way to B from C due to his versatility as a 2 mana card with Demoralize.
C Tier additions: Both Tidal Tsunamist and Portly Cannoneer fall to the C tier - I just don't find myself using these enough to justify placing them higher.
Let me know if you have thoughts on these and would rate any card differently.
@bjangles was right, this guy's the top Rare card in CA. Super versatile, and Silence makes him invaluable with Kelan being so powerful in today's meta. And Shapeshift will keep making him stronger over time.