Foretell is a simple, mechanically useful ability-- you don't have to play a lot of it for the cards to work. But it does get better in larger quantities due to the mystery factor. It would be logical to add a small-medium sized foretell package. It turns out there are quite a few efficient cards with foretell, so you don't need to play bad cards. Even the weakest section has some worthwhile options with cards like Dual Strike that have serious potential.
Good foretell cards:
Sarulf's Packmate, Behold the Multiverse, Ravenform, Poison the Cup, Glorious Protector, Doomskar, Battle Mammoth, Starnheim Unleashed
Medium Foretell Cards:
Alrund's Epiphany, Augury Raven, Iron Verdict, Saw It Coming, Demon Bolt, Mystic Reflections, Scorn Effigy, Vengegul Reaper, Dwarven Reinforcements, Struggle for Skemfar
Gods' Hall Guardian, Tergrid's Shadow, Kaya's Onslaught, Mammoth Growth, Dual Strike, Shepherd of the Cosmos, Return Upon the Tide, Depart the Realm, Haunting Voyage, Skull Raid, Doomskar Titan, Jarl of the Forsaken
Cosmos Charger, Vega, the Watcher, Ranar the Ever-Watchful
Also, sort of Alrund, but he comes later. Wow, that's a lot of cards for one set and one mechanic. Most of them are baseline decent, and the lords mean that you could even build it in as a real archetype (you'd also use Karfell Harbinger). I could also see a weaker cube built around Morph that also uses foretell cards to further lend to the mysterious play environment. I once theorycrafted a morph cube. It's not great looking back, but foretell could help liven such an environment up.
I wasn't sure where to put this guy. He's interesting, but probably all but unplayable.
Reidane, God of the Worthy // Valkmira, Protector's Sheild
Assuming the snow text is irrelevant, this is still a powerful card. The tax on the front half isn't that onerous, but it will come up, and the creature is decent on its own. The back side is not as relevant as often, but it can shut down aggro decks and token armies in their tracks. And it's nice to see a playable Urza's Armor effect. I think this card is awesome.
Tergrid, God of Fright // Tergrid's Lantern
Tergrid kinda wants to be at the helm of an archetype. The effect is insanely powerful, although the body is merely okay, and fails the Doom Blade test. The lantern is Torment of Scarabs plus upside, which seems pretty decent. If you get to big mana, you can start untapping it, and it starts to play a bit like Capsize. In the end, the card is too inefficient for most, but it's also super cool, and quite tempting.
Birgi, God of Storytelling // Harnfel,Horn of Bounty
Even without trying to storm off, I can see the mana production leading to some pretty walloping turns. The boast ability isn't going to mean much, but Centaur Courser stats are an okay base. The horn itself is pretty odd. You're paying five mana for this thing, and then it wants you to discard cards to get any benefit. But it's probably better than it looks, and being a random alternate mode means it's all upside.
Esika, God of the Tree // The Prismatic Bridge
She's kinda like a much worse Yavimaya Elder with a super cool RWUGB option. The double green is a bit awkward with what the card's trying to do, but I'm a sucker for this kind of thing.
This is on the list for stats alone. It also plays in a life-gain archetype, but I think that archetype is dubious in cube.
The backup mode is Assault Griffin, and a 3/2 flying for 1W is good. This is a weaker Bone Picker, and that card isn't super powerful to begin with, but it's not bad, and pauper legal.
I love it. Wild Griffin stats and an amazing ability that works across various archetypes all on a common. Delightful.
Well it misses cheat effects, but mass bounce is a big swing. I think I'm going to end up playing this.
2/3 for flying 2B with upside is new. This is sort of a sidegrade to Gorging Vulture.
Not the best, but downright okay, and pauper legal.
It functions like double-strike, and it can get double-strike on top of it. This guy hits pretty hard. Plus random tribal upside (+changelings).
Not the best hasty 5-drop dragon, but absolutely playable.
Maja, Bretagard Protector
There's no mystery here. It's a go-wide card. Great for an uncommon.
Sarulf, Realm Eater
This grows super fast. Usually it eats removal before you get to do anything too cool, but it's capable of some cool stuff.
Saves the creature from removal, and keeps the counter to boot. It's a powerful trick if you can spare room for one.
Seize the Spoils
Three mana is worse than two, but you get a shiny treasure!
Bind the Monster
At the end of the game, this can become uncastable, but one mana to tap and hold down a creature is worth a bit of life. It's not a blue Swords to Plowshares, but it is decent removal.
It's a selective four mana wrath. Better than Languish... eh, that's debatable, and it probably isn't. Still pretty decent. Black wraths are still not the best.
This might seem weigh down there on removal, but it's not necessarily worse than Disfigure/ Dead Weight.
In Search of Greatness
It's a powerful card. It's not exactly geared towards cube, but it's worth mentioning.
I don't really like this, but it's a decent b-list curve-topper. It needs to untap to pass the Doom Blade test unless you have some other way to gain life, but if it ever attacks or blocks, odds are you're getting a Serra Angel. It's a powerful, but fair finisher, and fine for weaker cubes.
Likewise, this is a decent card when you untap with it. The stats are not bad, and the ability is conservative, but abuseable to a point. It's a basic ol' b-list playable. Good with changelings.
She's worse than Fiend Hunter when you get down to it. Nonetheless it's still a decent card in a vacuum despite the various better versions.
Fynn, the Fangbearer
If you run poison, there are only so many options.
It's dubious red ramp. Strictly not bad.
Alrund, God of the Cosmos // Hakka, Whispering Raven
Man, for the god of the cosmos, this is one janky card! Syr Elenora, the Discerning was never very good, and this is sort of a side-grade. The foretell upside is barely even that effective when if you're playing foretell in your cube. The good news is his last ability is far from useless, and is often quite a bit better then Elenora's cantrip. Draw for lands to hit your six-drop, creatures if you need them, artifacts if you're digging for something specific, yadda yadda. Or use scries to get free cards. Hakka is even jankier than Alrund, but it has a few roles as an air blocker, an inefficient scry engine, and a source of etb triggers. It does none of those things well. The whole combination is really janky, but probably really fun to play, and better than it looks.
Who doesn't want to make icy manaliths?
Auras and Equipment Archetype:
This is nothing new in RW, but they've pushed the idea along a little bit.
Pyre of Heroes
This is an insane card in various formats, but jank at best for all but the most tribal of cubes. Nonetheless, the power is obvious, the interaction with changelings is great, and the card can work with a single-tribe theme-- for example a cube that supports allies or humans in various colors. Even there, it's jank.
Reflections of Littjara
As above. Crazy in some formats. Only borderline good even in a tribal cube.
It's efficient, but not the most exciting.
It's an early beater with an invoker-like ability that has a powerful interaction with the best green tribe. A good card to serve an elf theme, since it's useful early as a bear, midgame as a token generator, and late as an overrun. Not really worthwhile without a dedicated theme though.
It's not purely parasitic, since the 0 makes elves. But it's pretty parasitic. Much better than Nissa Revane though.
Better for most cubes than Tyvar Kell.