Goodbye hexproof: I already got rid of Plating, and now Veil is going as well. Although it's ostensibly important to protect big creatures, it's also a lot more fun to create a back and forth (and is frustrating when someone counters your big removal spell). White still has Valorous Stance, so it's not completely without protection.

The Enchantments deck is really fun when it's drawing tons of cards, so I relented and added another Enchantress (despite a new 1-of ability word). I risk adding too many parasitic cards only good in WG, but it's a deck that a couple people players really enjoy so I'm not super worried. Eidolon might also be OK in GU or something as well.

Still waiting on Tarkir previews to see if anything there fits.

One more thing: I've yet to see anyone use Lazotep Plating correctly for its Hexproof purposes. I think it's just got too much written on the card, so I'm exchanging it. I do probably need another Hexproof spell (You See A Guard Approach; Dive Down; etc), but an exchange spell is a bit flashy and seems fun so I want to try Trade first.

I'm happy with a few additions from Aetherdrift: Black has been looking for another good reanimation target outside of green (I love Whisper... but it hasn't been in a deck in forever); I'm excited for Magmakin to reintroduce a 3-card infinite combo; Wreck Remover is a perfect little role player; Rhet-Tomb Mystic is perfect as a 2/1 flying zombie wizard... that sometimes helps a cycling deck (less than it looks like on the surface).
Where I'm less clear is Marauding Mako (Hopefully more flexible than the 1/2 menace that demanding exactly cycling?), Aetherjacket (I never lived the Broom dream sadly), and Scrap Compactor (strict upgrade... but a lot wordier, which I don't like).

I also moved all the Landscapes and the newer 3-color cards to the Maybeboard. I haven't added them to the physical cube yet, and online it's disrupting tests; maybe I'll just swap in other 3-color cards occasionally.

Little here is certain.
Where Faerie was cooler but hard to answer, Mystic might be "easier" to answer, but overall stronger. Will depend a lot on player reactions, but I might switch this back.
Elas has seemed a bit too strong, but also isn't a payoff for lifegain as much as an engine, so I'm trading it while maintaining a drainer with the new Bloodwitch. The lion has been pretty complicated and also underwhelming, so trying out the clean anthem. Blight-Priest likewise hasn't seemed great, so trying out Tenacity (but might be too strong/annoying).
I've never been happy with Skola, so trying out Squirrel to see if it feels cooler.
Lastly, I wanted one more flashback card. Nibilis has always been decent if unexciting, hence its survival; I'll see if it ever comes back.

A mostly mouse-sized upgrade of a bunch of cards, with a couple main characters finally getting killed off.
I love the dream of Nucklavee, but no one else picks it, and it often leads to frustrating loops; Angler is just more in line with what the other UR payoffs are. (It also means I feel better bringing Cackling Counterpart back). I added a couple more untap effects (but removed Mirran Spy) to make the Knack/Helix thing a real UR payoff... we'll see how that goes.
Roar of the Wurm is great in theory, but a 4 mana 6/6 is not really worth the work to get it in the gy nowadays, and not being a creature hurts its place in other BG styles (like Spider Spawning). Testing out Thickest, though that might be too swingy.
I'm also swapping out the LOTR cyclers (except for blue) since they felt a bit too strong. They're borderline, so could make a return some day. For now, testing without them.
A couple notes among the minor upgrades/side-grades: I'm testing scry on Carrot Cake (it's still LESS complicated to read than Haunch); Instant speed removal is important, but feels bad at efficient rates, so I'm trying to shift to avoid cheap, unconditional surprises (e.g. Exorcise; Seal of Doom; Agate Assault) since at least when someone leaves 4-5 mana open and still has cards in hand, that's suspicious enough that you might avoid making a cool play that could get blown out.

Although most of the MH3 themes don't fit this cube, I'm still excited about some of the black cards we got. Mindless Conscription in particular might honestly be too strong, but we'll include it for now. And I'm shocked to be saying goodbye to Reassembling Skeleton.
Speaking of too strong & cutting a mainstay, I've finally come to terms with Deep Analysis: It is not fitting my objective that powerful cards look the part, and it has saved me from so many losing games (or put me way over the top) even though the other players think they really lost to Deeproot Champion or whatever. There's enough card draw in blue that I'm trying another tap effect that can still draw a card.
As for what's leaving: Dire Moon just slows the game down a lot: vs a big & small threat it can't attack, but that also means the opponent also keeps their big threat back to block, creating big stalls. Hopefully Larva being 5 mana and bigger will help this phenomenon despite creating another small deathtoucher.

Lastly, I'm excited by the landscapes but I'm not sure how to get land counts appropriate, so I'm just going to experiment and add 15 new cards (1 pack): all 10 landscapes and the 5 3-color combinations I don't have. The latter especially presents some risk since those tend to wheel a lot more, but they can also be exciting so we'll see what sticks.

After sitting on this a while, the Varmint is the only card I'm adding. Strong consideration for Reach for the Sky, but will need to ponder a bit more about its replacement.

I almost forgot about this swap. Creating an army of 4/4 Nucklavees with Counterpart is fun for exactly one person exactly once. Counterpart is also secretly one of the best blue cards, but doesn't read like it, so I'm cutting it for that reason as well.

Boards still get pretty cluttered, and my playgroup seems to enjoy green, so I'm adding a couple more removal spells to make sure everyone gets their fill. (Also, Sailback just sat in folks' hand until they saw an artifact/enchantment, which feels bad when they don't). Panther gets in because "it's cute!" and I can't decide if the reach on Ancient has been necessary or backbreaking.
I opened a Melek, so testing it out instead of the other newcomer (seems more exciting for someone to play)
I'm also testing the enemy bicycle lands (again) to see how it plays out instead of just hoping they print them someday. The current suite of enemy lands is just pretty unbalanced in terms of power, and I'm honestly OK slowing down RW.

I thought the Thriving lands would be a great, newbie friendly solution to manabases: if you're one of those colors, take it and play it. Instead, they've been a little confusing and involve memory issues, so they've mostly benefited my off-the wall 4-5 color decks that don't need help with the addition of LOTR landcyclers (1-mana has been HUGE for speed). Until I figure out something better, these are switching back to a variety of artifacts (including perfect newcomer Runaway Boulder!)
Seismic Monstrosaur is perfect. The other landcyclers were decent but not enough to replace what I have.
Beanstalk I'm unsure about; might not work well, but testing it out. Could be Cease//Desist? (which is kinda green, right?)
Planar Cleansing hasn't been exciting and doesn't fit the W decks, so testing out a more straightforward removal spell.
I'm also writing "Zombie" on Virus Beetle, haha.

Lastly, at our most recent draft I randomly had in Pierce Strider as an early Thriving land replacement... and someone combo'd out their opponent with that and Escape Protocol, so I'm officially leaving it in (for now).

Somehow "Remove/Replace" didn't get clicked.

In an effort to reduce hard to answer "win the game" cards, Oran-Rief Ooze is just too strong. Enough of the "+1/+1 counters matter" cards are self-contained and UG draws enough cards that I don't think the theme needs more help. I'm also excited to push UG more towards "heroic" (without the keyword) since it melds well with GW and helps give purpose to other cards I want in the cube, like Banishing Knack.
What does need help is UR, which is getting 4 new cards. I just realized it's the only color pair still with both hybrid cards, so it's getting an upgrade that doesn't need to be played before all your spells. I'm also trying Song and Alchemist - both of which could be too strong - instead of 2 cycling payoffs: Adept (which everyone kept playing without being super aggressive or having any ways to discard) and Stinger (I lose a Wizard, but had little purpose otherwise). Relentless Advance was too weak before and might still be, but it's simple and enough of a mild role player in a few decks that I think it's worth trying again.

The Drake Haven switch has been good (so far), so I need enchantment removal much less. Other options for this slot besides the Bloodline were Up the Beanstalk, Invigorating Surge, and (from a previous post) still Mother Bear, Garruk's Companion, and Dreampod Druid.

Lastly, I can still make 4-color decks with ease, so the mana might be too good... but folks also might not be taking lands highly enough. I'll be considering cutting the Thriving lands in the next update until that trend changes.

I decided Collector's Vault was worth trying as yet another mana fixing, card drawing artifact (so hopefully ok in RB, UG, ramp, etc.) that's yet another Ocalchase Daredevil combo. Scrabbling Claws is a nice gy answer, but gy strategies haven't felt oppressive and there are other answers, so claws only looks good alongside Furnace Celebration or something.

I finally got to see Spiteful Prankster in action, and it was super obnoxious for all of my opponents - Definitely MVP - so I'm downgrading back to Iguanar, which will hopefully be easier to deal with (and not as potent in combat). It turns out boards get pretty messy when everyone's trying to setup their engines, so abilities like that on Iguanar and Falkenrath Noble are very hard to overcome and even see coming (just the sudden realization that combat will result in your death); "you control" will be a good line of text to look out for to replace these with as well.
Likewise, Stone Haven Pilgrim can feel oppressively strong when it's turned on early since it's both a 3/3 for 2 and turns off the ability for the opponent to race. Opting for the simpler 2/2 Lifelink.
Lastly, only a couple of Eldraine inclusions, sadly (especially for an enchantment themed set!): Flick a Coin is exactly what I would've wanted when I added Improvised Weaponry; Sprite is an experiment, and even though I just added Recon, it's not always obvious to people when to put that card in their deck, so it's still the most flexible Blue slot.

At the risk of angering purists who think other IPs don't belong in Magic, I'm adding the new Landcyclers and a couple of other decent cards. Seems fitting that green gets 3 landcyclers. Legolas seems possibly too strong, but worth a shot (I also can't remember the last time Commune made it into either GW or GB). Rosie seems really cool, since tokens are everywhere (>1/6 of the cube makes a token) and will be fun to discover different places she fits. With the previous addition of Elas, 3 static death->drain effects are a lot, so I'm subbing Cutthroat with Return to Action (definitely weaker, but still lifegain related and can lead to some fun moments).
Lastly, Visionary is great but barely fits one the GU theme (plus green is becoming really strong already). There's been an appetite for griefing other players, so I'm trying out Kavu over options like Mother Bear, Garruk's Companion, and Dreampod Druid.
RIP Eternal Dragon.

I didn't realize Alandra existed, so testing out the swap with Drake Haven. Pros/cons to both. Might switch them back; we'll see.

(See previous post)
Also, I want to try out the Rottentail, both as a super cute card that could be a good crossover between GU and GB (and maybe GW given vigilance, but unlikely).

[Comments are for this and the next update, while I wait for the full spoiler to make its way onto Cube Cobra]

Another test draft - plus a feeling I've had from my online tests - revealed that WU fliers just can't really keep up. It's 2 mana 2/1 fliers look attractive enough to everyone that it's hard to build a fast curve, and there isn't enough late game inevitability against the slower decks. I'm trying to offset that with a couple cheaper fliers, and more instant speed options. I'm torn between Skyfisher & Loyal Pegasus, but trying the former for now. Reconnaissance Mission can hopefully help refuel, and could also be good in other decks (a zombie swarm, G's trample, etc.). This might not be the end of buffing fliers either.

MOM also brings new landcyclers (to be added to the cube in Pt2)! The playtest showed that folks aren't even playing the on-color landcyclers they get, and any multicolor ones don't look attractive when they're not in those colors (even though in my mind it feels like this cube's fetchland equivalent). I'm hoping that switching them to mono-color, as well as being better (yay power creep?), will help folks actually choose to play them (although the red one is identical to Lava Serpent, so gotta shift that). Despite not playing them, everyone's mana was still ~good~ OK, so I feel OK dumping the Crystals (which were offsetting the balance of the "dual" cyclers anyway). Artifact synergies are already overdrafted, and my cycling decks have had plenty of cyclers, so I don't think I'll miss having net fewer cycling cards (-7 +2 besides trading landcyclers).
Yargle also went around the table to no fanfare despite 3 of the 6 decks having black. Sorry Yargle, maybe another time.

I realized it's rare to naturally navigate into Zombies besides just starting with decent Black or Blue cards that also happen to be zombies, so I'm trying out Death-Priest as something more universally decent (good in BW, BR, and obviously BU) and the more exciting Shambler to improve the curve. It might need more than this, particularly in blue, but something to watch for.
I want more simple/charming cards that are still evocative, so I'm trying Yargle even though he has nothing to do with any strategy.
Reducing the amount of text walls by duplicating Hawk instead of Cleric. It does mean I need more enchantment removal instead, but I'll keep an eye out.

Last fall I did both a test draft with someone new to drafting (where I observed their picks on here), and then finally had my first post-COVID playtest. Both revealed how overwhelming a lot of the cards can be, especially when it's on top of so much else; and how they can be misconstrued as to when to use them (making them traps). But it also revealed how cool, complicated rares can still be worth learning, or create great stories around the table (looking at you, Sharknado). So, I'm taking a first step to cut that down at least a little so there's less to process, and there are fewer niche trap cards. Cycling in particular was more confusing for folks than I realized it would be (it's often hard for someone new to decide when to cycle). Instants as well are not easy for newer players to time appropriately, so try to avoid ones with specific timing requirements.
I also need more incidental removal, especially for enchantments. There are some strong enchantments in the cube, and several removal spells in the playtest ended up in sideboards or in the deck that had the must-kill enchantments. This isn't completely solved this update, so something to watch out for moving forward as well.
I also don't think I should be afraid to run duplicates of cards that are perfect for their color combinations.

Lastly, all of these are cards that can come back once I'm more confident in audience comprehension.