A plane that no longer moves. One side is completely frozen and the other is a hellish wasteland where the sun never sets.

Card Type Matters (not subtype, I'm talking the stuff Tarmogoyf cares about)

Enchantments vs Artifacts / Religion vs Technology
Diversity vs Quantity
Instant/Sorceries vs Permanents / Short term vs Long term
Etb vs Ltb
In play vs In other zones
Constructive vs Destructive (Untap/Tap, Search/Destroy, Life = #/Damage = #)

I think a majority of the set shouldn't encourage people to go for every card type at once. A couple "domain"-like cards (like from the mastery proposal) at higher rarities is fine, but I don't think it's interesting at all as a major theme--players play with a mix of card types in every environment, so in the end the "card types matter" theme ends up just being "make sure you have a couple artifacts and enchantments." I think the far more interesting and unique direction to take is allowing/encouraging players to focus on one or two card types.

Think about Lorwyn--it doesn't achieve the tribal (i.e. "creature type matters") theme by encouraging people to use a mix of every kind of creature, it does it by dividing the creature types into a few overlapping subsets with unique identities and playstyles. It also doesn't do it by being solely a "treefolk matter" set--obviously it's different when our factions aren't just different kinds of creatures, but I think a pluralistic theme is perfectly viable.

Also keep in mind that a deck doesn't need to consist entirely of a single card type for that card type to "matter," and that not being focused on creatures doesn't mean creatures can't make an appearance. There are artifact creatures, (probably) enchantment creatures, tokens, and permanents that turn into creatures even if they aren't generally of that type. There are supporting creatures like Frostblight Scavenger and many more to come. There can be creatures that are meant for combat, but still depend on other card types, like Kiln Fiend or whatever. Or someone could draft a strong "instants matter" deck with removal and counterspells and recursion and just run a couple unrelated creatures for win conditions. I count all of those as victories for the set theme.

Anyway, from my perspective, we're not looking for a keyword that tells everyone to play every card type. And making a separate keyword for each of five or six card types is probably ridiculous. I think our best bet is something parametric like kinship that could go with a number of different card types.

While it was sort of the point of the criteria, we should probably discuss how to differentiate the two/three environments (because your cards aren't telling me).
Enchantments vs Artifacts / Religion vs Technology
Diversity vs Quantity
Instant/Sorceries vs Permanents / Short term vs Long term
Etb vs Ltb
In play vs In other zones
Constructive vs Destructive (Untap/Tap, Search/Destroy, Life = #/Damage = #)

Sunny side: Illpyre
Frozen side: Frostwynd

Both sides are probably pretty bad. There should be nomads all over each side, as well as people that hate it on their side and make the long journey to visit the other. I like the short term vs long term idea. On the hot side things would rarely last long, and on the cold side things would tend to slow down and stagnate.

I think enchantment creatures are necessary if you want a "type matters" theme to actually work in limited.

I think enchantment dudes should be r/b. Thus white should be enchantment hate/artifact.

Does anyone else think it's weird that red loves sorceries and blue loves instants most of the time? Like, counterspells and trickery, I guess, but it seems out of character for red to be the color of slow, deliberate spells.

I'm starting to think Snow shouldn't be supported in this set since it's not a card type.
Thoughts?

Just thinking out loud here, would we want to push equipment/auras on both sides. A way for citizens to cope with the unnatural weather?

We have 5-6 factions: artifact, creature, enchantment, instant, and sorcery at the very least, and maybe lands?

The whole instant/sorcery divide is goign to be annoying. Tempted to just erase it and make instant a supertype or do something with flash instead.

What's the middle area like, the terminator, like? We have to address it somehow. Is it a verdant paradise for everyone to fight over, or do the weather effects turn it into a hellish wasteland combining the worst of both worlds? Maybe a change in thenature/location of the terminator could play a part in the story?

White/Blue: Sorceries
Blue/Black: Artifacts
Black/Red: Instants
Red/Green: Creatures
Green/White: Enchantments

Just a thought.

Blue should definitely be involved with instants.

I was hoping to somehow make red be associated with enchantments, since I was under the impression the sun side would be enchantment, and the sun side is obviously red.

The biggest problem with this set that we'll have to solve, I think, is how to distinguish it mechanically from Mirrodin, Theros, and Zendikar with regards to artifacts, enchantments, and lands, respectively, and have it all come together as a cohesive unit rather than a themeless smorgasbord.

I was hoping to somehow make red be associated with enchantments, since I was under the impression the sun side would be enchantment, and the sun side is obviously red.

Splitting the color pie in two and distributing them between the two sides seems lazy to me. I think it's more interesting to show how a color adapts differently to different areas.
It's some sort of pet peeve of mine. Split the colors into regions and I split your torso, or something like that.