You'll notice some significant changes outside of just those listed here, that's because I hate clogging the blog. Here's a rough list of the changes for archetypes:

• RG is now Escape, their overlap is graveyard stuff from green and sacrifice stuff from red.
• Prowess is now Prescient (the same thing but you scry 1 after the buff applies); this mainly appears on red cards, so now Heroic overlaps with Scry significantly.
• Cards that used to look at the top of your library now integrate scry directly. There were so many cards in green that did this but didn't scry, so I've changed them.
• Multiple Constellation cards from blue have scry stapled onto them.

The bulk of these changes are focused on the creatures. If I can get the creature counts balanced, the noncreature side will be much easier to keep up. I did a tally of the creatures that actively contribute to each 2-color archetype, and I found (unsurprisingly) that UR, UG, and GR were the least supported with creatures.

Now, I obviously could have just added a bunch of creatures that only synergize with scrying and removed a bunch of heroic, constellation, and other stuff. Instead, I decided to modify the cards I already have to work with scry and escape. Prescient goes a long way towards that. I want to have a critical mass of scry such that every archetype can use these cards as just generically good curve-fixer cards.

The biggest thing that informs this change is that I have access to 8 players more consistently than I have 6 players. I brought the cube down from 360 to 270 to reduce the amount of custom cards I'd have to make, but I want more opportunities to play this cube than I currently have.

The other event that informs this change was playing a friend's all custom JJK Cube. 248 unique designs, commander draft, surprisingly extremely well-balanced. This experience has emboldened me to be much more daring with the changes I'm willing to make to cards. If the goal is to create a set that truly feels like Theros, both mechanically and thematically, then I have to be willing to step out of what's normally available in standard card designs.

In order to expand the cube from 270 to 360, I plan on being much more aggressive with changing the rules text of cards that don't fit the cube's mechanical needs. I will need to mark the modified cards in some way other than "This card has been modified" - maybe something in the corner, not sure yet.

For ease of use, I've made a modified tag with a color that reminds me which cards have changed.

This playtest went extremely well. We played with 6 players, and the overall feedback was extremely positive. The following changes are to address the lack of solid targets and spells for heroic and the replacement of one card (warehouse tabby) with a decent removal spell that flips into a creature engine (notably, an enchantment).

The cube in its current state is at around 180 cards native to Theros, another 180 outside of Theros, meaning it's become extremely weighted towards non-theros cards. To combat this, I'm going to cull the entire cube down to 270 cards. I'm expecting around 170 native Theros cards, around 100 non-native Theros cards (which, when you remove the ~40 dual lands that don't need to be on Theros, having 60 nonland non-theros cards seems about right to me).

Focusing as I said on culling any cards specifically from outside of Theros that I don't think are necessary, as well as most of the non-creature non-enchantment cards. With less space it's all the more important to make sure that the core of the draft (creatures) and the core of theros (enchantments) stay consistent.

It's clear that the category that takes the most away from the creatures is the enchantments, but cutting enchantments is just against the spirit of the plane. So I'm preemptively adding a number of enchantment creatures or creatures that become enchantments.

First playtest was rough. In building the cube I was so focused on making sure that there was sufficient enchantments that I didn't think to ensure sufficient creatures.
After some long discussion with the players, we diagnosed three primary problems with the cube, which I will attempt to resolve here:

• Magic has changed since 2013. Cards like Mantle of the Wolf and Archon of Sun's Grace, which seemed innocuous to me, were completely dominant and often immediately won games.
• There were not nearly enough creatures. 3/4 decks used out-of-synergy creatures just because they were the only creatures available.
• 5c Good Stuff trumped synergy decks hard. Due to the lack of creatures, the format was slow enough that the 5c deck could afford to wait a little bit and drop its bombs.

Solutions:

• The cube's power level is getting dialed back a bit to the general power of OG Theros' rares and mythics. Cards that win the game easily without the need for synergy are being replaced with cards that are just as powerful but still require synergy.
• As you can hopefully see, nearly every card being added is a creature. We're looking for something near 50% of the cards in the cube being creatures, if not that then at least 50% of the non-lands.
• We're now breaking singleton to allow the cube more flexibility. At the moment, this is only applying to the creatures in the scrying synergy. In addition, a number of god cards that weren't particularly synergistic have been replaced with more interesting variants.

As you can see, we're taking out a few cards and adding back a ton more. The goal here is to add and remove until we have relatively consistent power levels across the archetypes. Once we have that, we can start to trim down. Here's some basic notes:
Scry needed more creatures, so I added some creatures that care about the top card of the library.
Blue needed more / better heroic spells, so I added a number of auras and targeting instants/sorceries that I can choose from later.
Inspired is getting cut as a supported mechanic. It's good if you have Phenax or Thassa's Ire, but it's bad everywhere else.
Mill/Graveyard had barely any support in blue. With Inspired gone we have room for cards like Ashiok, Dream Render and Thief of Sanity, as well as low-cost mill cards in blue.
Sagas & Seals have been added. These might be harder than I expect to reskin, but mechanically I want enchantments to be doing the work that sorceries are currently doing. There are some sagas that just do that best. Of course, I'm also adding the entire Seal of X cycle, because they're all amazing compared to the fonts. And I don't really have to reskin them :)
Finally Heroic needed some trimming and shifting. I went through the creatures and cut some of the weaker ones, then I sought replacements for the worst of the targeting spells, hoping to give these decks some consistency.

This was originally a big theros set cube, attempting to simulate a draft environment made up of all four theros sets combined.
This is completely different from that. This cube has the goal, among other things, of offering a mechanically consistent and interesting draft experience. For this reason, many cards will be added that are not native to Theros in art or flavor. These cards will be reskinned with new art, names, flavor text, type lines, and even rules text where necessary. More on that later.
For now, we need to make sure that the cube runs efficiently, and that means cutting chaff. For each card, I will identify with its tags which deck archetypes it fits into. From there, I'll do passes of sorting by tag. Archetypes that have too many cards will see cuts, those that have too few will have cards added (likely from outside of Theros).