Long overdue!
IKORIA:
In the past two months, Ikoria completely overhauled every Constructed format thanks to companion but none of us have been able to Cube with Ikoria cards in paper. That puts these evaluations in the odd position of having no direct experience to inform them but inevitably taking the impact on Constructed (Standard/Pioneer, for the non-companion cards) into account
Mutate:
Mutate looks very complicated and is... still pretty complicated, but it's mostly fine? If you have a playgroup that's experienced and likely to play with these cards again and again, this upfront cost may be worth it. Explaining the arcane rules interactions to that more casual friend who's making up the numbers for this week's draft might scare them off for good
I like that Mutate asks you to think carefully about what types of creature are good to mutate onto and which abilities work especially well with this Mutate card. It also has a well-tuned degree of parasitism: you can play a single Mutate card as Bestow+, as many as you can find to spam the Mutate triggers, or one or two that will lead to some big swings sometimes if you can pair them together
The rare cycle is a promising experiment with hybrid mana that I hope we see in other contexts
Illuna, Apex of Wishes: A fine way to get the dopamine rush from Cascade with no other mutators in sight, good stats; it's funny that this card shows up barely or even not at all in the Standard Mutate decks that can cast it
Nethroi, Apex of Death: An excellent card if hardcasting it is ever sensible and useful
Snapdax, Apex of the Hunt: We finally have a playable Mardu card and all it took was not needing to be in all the Mardu colours!
Vadrok, Apex of Thunder: More interesting as a much-needed form of variety in Boros than anything else but pretty good at that especially in Cubes around these parts; likely use case is as a build-your-own Goblin Dark-Dwellers with Hordeling Outburst or similar?
Sea-Dasher Octopus: The best variant on Curiosity in mono-U and an excellent pick-up for a creature-heavy flash deck. Oddly good in green between Birds of Paradise/Gilded Goose/Ice-Fang Coatl, trample, and big creatures that gain a card whether they connect with Octopus or eat a chump blocker
Dirge Bat: Hasn't showed up at all in Constructed but may have a brighter future here - a nice twist on Nekrataal that doesn't get blinked until you want to stop playing
Everquill Phoenix: Phoenix tribal is a fun pipe dream to shoot for - fine on its own merits too!
Gemrazer: Love this one - I want a certain density of Naturalize effects and I'm much more into Thrashing Brontodon and this than Reclamation Sage. You'll definitely get someone with the reach on this at least once
Parcelbeast: Enables some great starts but mostly in draws that were already good - probably not worth it unless you can Mutate it on T2 reliably and maybe expect more Mutate triggers
Regal Leosaur: White and red could both use a Trial of Solidarity a lot of the time so this is an appealing hybrid though it opens you up to spot removal that's typically not as strong there
Companion:
Since I should have wrote this up, companion has had such a pernicious influence on every Constructed format that it became the first mechanic AFAIK to require a retrospective fix. For Cube the non-Lutri deckbuilding restrictions are much harder to meet and an interesting goal to shoot for in drafting so I'd rather play with them as intended (not as printed since you can't divine how they are meant to work just by reading the card, but that's another topic...)
Lutri, the Spellchaser: Free in a lower % of Cubes on this website than anywhere else in the Cube community and possibly worth excluding just because it requires no effort; that said, this will lead to some very fun turns so maybe that's ok
Umori, the Collector: Even the most creature-heavy green decks want to make use of the non-creature enablers that set them up so Umori is still a significant cost and the companion criterion is so hard to meet
Gyruda, Doom of Depths: Condition is challenging but rewarding - blue has a lot of good even-CMC Clones, blue and black both have more great 4-drops than you can include, other colours have solid version of every staple effect up and down the curve
Yorion, Sky Nomad: This would be an auto-include for me sans companion so the extra potential makes it a slam-dunk. +20 cards is easily achievable but you really have to focus during the draft and be willing to make sacrifices for it. Yorion is deserving of its own writeup so I'll do that separately
Lurrus of the Dream-Den: Another fantastic Cube card with bonus companion text - this one is more straightforward than Yorion and there's not much to say beyond listing every CMC <=2 permanent in a list. It speaks for itself!
Lavabrink Venturer: The odd/even choice means that you can ensure you're immune to specific things but they can probably still interact with this somehow, which is the ideal spot for a card like this to hit. If you want to enhance it with something that dictates the value of your choice - the 'equip Jitte to True-Name Nemesis' lines aren't so easy here
Luminous Broodmoth: I haven't heard much follow-up to the initial hype over Nightmare Shepherd and this is in a colour less conducive to sac strategies; despite all that, I have higher hopes for this? Keeping the card instead of a copy, having the full power/toughness of the original, and giving them flying means that Broodmoth is great at applying pressure even through a sweeper and if the game goes longer you can set up more elaborate Broodmoth sequences with other rebuy effects
Neutralize: I don't like Cycling on a card this generic on principle but a fine replacement for Sinister Sabotage, Disallow etc if you care about the mechanic
Ominous Seas: Not to be confused with Omen of the Sea (though maybe to be confused since it's in the very next set?!), it's hard to gauge how easy this will be to activate in the average game. Without a decent number of 'above-rate' draws like Brainstorm or Sylvan Library (though having a lot of cyclers etc makes it easier to find those), this is more of a Suspend ~5 8/8 - not the worst?
Shark Typhoon: A home run - an excellent, interesting card in its own right, ties into various themes, and fun in all of its modes. It's tough to imagine a blue deck that doesn't want this but it's more engaging than the broken blue cards that's also true for
Extinction Event: An original twist on conditional sweepers in black - easier to build around than some and guaranteed to hit their biggest threat even if you have to lose material in the process. Notably, 'even' always hits tokens
Heartless Act: The most powerful Doom Blade variant because it's the most universal - more true for
me since most of my Cube sketches have a lot of artifact creatures and legends that dodge Go for the Throat/Cast Down
Fire Prophecy: If your red section is mainly aggressive, the bar for any burn spell that 'only' hits creatures is very high. That said, even in that context Prophecy can cash in a one-drop that stopped being useful turns ago for another shot at a game-ender while removing a blocker and it's obviously aces in midrange or control. Tucking a card is a rare effect in red that the half-dozen Polymorphs in this batch of releases
Mythos of Vadrok: More convincing as a midgame Plague Wind for 2RR than a future-shifted Aurelia's Fury
Weaponize the Monsters: Likely worse than Goblin Bombardment and that highlights just how critical a sac outlet being free is; that said, a Cube that really wants one Bombardment could easily want a second and this one should play out differently enough that it's a good complement to it
Yidaro, Wandering Monster: A rare red finisher with an interesting subgame and a way to recycle itself for the Polymorph crew. I like that if you want to reanimate it you have to find some other outlet
Kogla, the Titan Ape: Variants of this are a common custom card in green so it's nice to see it actually in print and offering some useful variety to green's midrange roster. The Human bounce effect is a bit out of left field and I'm unsure how often it will actually come up but there are a lot of great Humans to loop with it
Migration Path: Welcome and unsurprising but is removing some of ramp's inherent inconsistency actually a good thing? It's so easy for ramp to be the best strategy in a format thanks to stuff like this
Migratory Greathorn: A promising card even without any other mutators in a creature-heavy, mana-hungry deck. These twists on staples help to give Cubes an identity without making a large commitment
Vivien, Monsters' Advocate: There's a ton going on here that makes this more compelling than most of the 5-mana Nissas or the equivalent planeswalkers in other colours. The Vizier of the Menagerie text and the tutor ability pull you in subtly different directions and I like that this is a 7.5/10 that you can boost to a 9/10 if you try
Wilt: A slam-dunk if your Cube has... artifacts or enchantments in it
Chevill, Bane of Monsters: Competition among BG 2-drops is strong (though I typically hate evaluating new cards in those terms) but the ceiling on this is as high as any of the others in a creature-heavy format
General Kudro of Drannith: A fantastic lord for the WB Humans deck written about so well
here
Kinnan, Bonder Prodigy: The wording here is unfortunately finicky (you really want this to work with Urza and you'll be really disappointed!) and the very conventional Elf-heavy green sections that want this tend to be more conservative with gold cards (UG going from maybe the worst guild to one of the best in a year didn't help either)
Narset of the Ancient Way: Has only been underwhelming according to most reports I've seen
Rielle, the Everwise: Read shamizy's great writeup
here for the details but I'm excited to finally have an appealing build-around incentive in UR
Winota, Joiner of Forces: Same but for WR
Skull Prophet: If you want a BG card that isn't yet another removal spell and you support graveyard themes, this is a very workmanlike candidate
Fiend Artisan: Scratch that, this one is a lot deeper! Zombie Pod has always been a fun Cube transplant from Standard so having an in-colour threat that also sacs and searches for creatures is brilliant
Skycat Sovereign: WU fliers is an increasingly well-supported theme and this is a better lord-of-sorts than the likes of Empyrean Eagle
Sprite Dragon: The spells-matter two-drop that UR actually needed! It's hard to make the counters matter for more than their permanence in UR but if you're into Volt Charge et al. this makes them even better
Crystalline Giant: Held up as the example of how sets are designed with digital play in mind but I don't think this is at all hard to track with a modicum of effort? A neat way to incorporate in-your-face variance into the game and a nice artifact threat to add to that roster. An underappreciated card IMO
The Ozolith: Truly, truly berserk in a +1/+1 counters deck so it depends how strongly you want to push that; however strong it seems, it's better than that
Triomes : I recently pondered redesigning my land base around the AKH cyclers like Sheltered Thicket with fetchlands and the Triomes have opened up a whole new horizon there. I'm very tempted to proxy the Shard Triomes and make that the default model for my Cubes going forward
COMMANDER 2020
An odd set with fewer treats for Cube than most of its siblings. None of the gold legends really tempt me and some otherwise promising cards are worded for multiplayer in a way that renders them much weaker or useless for 1v1. That said, there are some useful niche cards:
Agitator Ant: An interesting card against a creature-light deck but most Cubes these days have enough creatures across most colours that this will backfire a lot and I don't know if it's worth it.
Fireflux Squad: The Polymorphs just keep coming! Red has enough good token makers that you can play this in a normal-ish deck and freeroll an upgrade to your 1/1 Goblin or Elemental or something but is also a solid Polymorph in a deck with stuff like Lukka, Sneak Attack, etc. I usually point out the utility of having this effect attached to a creature but a deck that's able to use this most effectively won't have many other creatures to Survival for, Reanimate, and so on. However, the rate on this card used 'fairly' is just fine - 3R for a 4/3 haste that upgrades your worst creature into another immediate attacker. That makes it a fine card for go-wide aggro decks, a popular direction for red. Having these Polymorphs that are good 'normal' cards makes it easier to support the ones that aren't
Surly Badgersaur: I LOVE THIS! A wacky but exciting discard payoff is just what I'm looking for. The lack of additional costs is a big deal as you can chain discard + draw effects together on one turn (especially when you're making Treasures) and a card like Drake Haven has an in-built ceiling on how bonkers it can be as the enablers can be quite mana-intensive. Also a perfect curve-topper for the aggro or midrange Madness decks in RG
Glademuse: A potentially dangerous symmetry here vs instant removal but a better centerpiece for a flash deck than all the recent cards that do that more earnestly. UG Flash is an established and solid archetype but WG Flash has a lot of potential as I discussed in the OP
Ravenous Gigantotherium: Another solid finisher with a safeguard against being too
good if you cheat it into play before you should, you naughty __
Sawtusk Demolisher: I like the use of Commander sets and the like as a way to push Standard mechanics in ways that wouldn't be ok there. This is a fantastic incentive for playing cheap creatures - if you have enough of them, upgrading one to a 3/3 makes your investment even stronger
Slippery Bogbonder: The flash/counters crossover I never knew I needed? If you aren't feeling ambitious you can take the guaranteed hexproof and load your counters on this, which makes it a lot more reliable
Manascape Refractor: One of the coolest mana rocks - finally, Bazaar of Baghdad actually makes mana! - but I doubt this will come up often in practice
Nesting Grounds: Doesn't specify the type of counter so there are lots of corner cases like endlessly resetting Sagas