I've been excited about Foundations since it was announced, and even a terrible record with a middling-bad prerelease deck couldn't dampen my enthusiasm (BW midrange, not enough removal or lifegain payoffs). While I didn't ride to victory in battle with Magic's new variation on a core set, I did open a lot of generally useful cards for the cube.

White
Healer's Hawk is a solid evasive 1-drop with incidental bird and lifegain synergy for those developing themes. Sun-Blessed Healer and Cathar Commando are both flexible, aggressive cards with bonus utility, and Dauntless Veteran adds some more support for go-wide strategies. Inspiring Paladin is the most interesting new White creature - useful on its own as a 3/3 First Striker for 3, and getting a second +1/+1 counter lord into the cube should help that theme coalesce into something you can reliably build around. To help with that I also added the new Felidar Savior (a strong card in its own right) and Armasaur Guide, and brought Tempered Veteran in from the maybeboard. Angel of Finality is mostly here as a 3/4 flier, although the possibility of nuking a Reassembling Skeleton will occasionally be relevant. I'm slightly less jazzed about Vanguard Seraph, but maybe the extra surveil will surprise me.

For non-creature spells Fleeting Flight packs a lot into a single 1-mana combat trick, and White picks up two more removal spells in Stroke of Midnight and Luminous Rebuke.

If it wasn't already true, I think with this update White is probably the strongest color in the cube at the moment.

Blue
Elementalist Adept is a new and much-needed 2-drop for Blue, and along with Mocking Sprite and Tolarian Terror the cube is finally starting to have a little bit of a proper spellslinger theme in U.

Icewind Elemental seems like a solid common and provides another Blue target for Lilysplash Mentor. I'm trying Curator of Destinies out over Sphinx of Uthuun, mostly because it's new. They're pretty similar, so I might not notice a huge difference here.

The only non-creature spell for Blue in this update is a copy of Imprisoned in the Moon. I may add Witness Protection later as well if it seems like Blue needs more removal.

Blue is a bit of a janky color in this cube, but I think the new cards are pushing it in a decent direction.

Black
Black gets a big update this time around. Infestation Sage, Vengeful Bloodwitch, Vampire Soulcaller, and Eaten Alive give my (previously somewhat anemic) aristocrats package a nice boost, while Tinybones, Bauble Burglar and Pilfer add a little discard theme. I brought Liliana's Caress in from the sideboard as well, and am considering adding a Duress.

Gutless Plunderer is kind of a weird one, but 2/2 Deathtouch with upside for 3 is good enough given my limited pool of Black cards. Billowing Shriekmass is very fun and makes me wish that I had more Threshold payoffs to build around, especially given all of the loot effects in Blue and "ring tempts you" in Black.

Black's lifegain got some more cards as well: Sanguine Syphoner triggers a lot of things, Midnight Snack provides even more food, and Exsanguinate is... fine. With Crypt Ghast in the list and no creatures over 5 mana, Black needs something to do with its mana in the late game.

Red
Red's pickups are unexciting but important. I was pretty light on cheap aggressive creatures, so Fanatical Firebrand and Frenzied Goblin are nice to see. Axgard Cavalry is workable (and another step on the way to a workable RW Valiant theme), and Courageous Goblin is a cute addition to the RG Power Matters theme. Firebrand Archer convinced me to add Kindlespark Duo in from Bloomburrow, so Red now has a good density of spellslinger payoffs. If only Red had some decent spells to sling...

On that front, Burst Lightning adds another desperately needed burn card. I think Incinerating Blast is probably pretty bad, but beggars can't be choosers and I don't have a lot of Red spells in my collection to pull from.

Green
The big one here was opening a Doubling Season in my one prize pack. Now you can create even more food! Fortunately I also got some good +1/+1 counters cards in Nessian Hornbeetle, Cackling Prowler, Apothecary Stomper, Snakeskin Veil, and Felling Blow.

Beast-Kin Ranger is really interesting to me, and I think he'll be fun to play in a variety of decks whether you're setting up a Reassembling Skeleton loop, durdling around with Lilysplash Mentor, or playing Hop to It for a big attack. I've added in Presence of Gond from the maybeboard, and Elfsworn Giant as a strictly better Sporemound.

Continuing the solid 3-drops, Ambush Wolf seems fine, and I think it's funny that it's in the list with Ambush Viper. Eager Trufflesnout adds even more food and finally some Trample that doesn't rely on Crowned Ceratok.

Gold
Elenda, Saint of Dusk was the bomb rare in my prerelease deck, and I was happy to see that she's both very strong and not unbearable to play against. Tatyova, Benthic Druid doesn't really have the support to pop off in this cube, but I think she's generally good so I was happy to add her to Simic. And hey, maybe you can flicker your Dryad Arbor for repeated landfall triggers.

Grey
Gleaming Barrier is probably the best new addition here for how long it can blunt an aggressive deck if you get it out early. Juggernaut is kinda bad and Solemn Simulacrum is fine. We also added a Swiftfoot Boots to the equipment suite, so Shrike Force and Fencing Ace are feeling pretty happy.

I got a Tranquil Cove and a Dismal Backwater, so now I almost have two lands for each guild pair (Gruul is still using Savage Lands as its 2nd land). I added a Rogue's Passage for utility and a third copy of Evolving Wilds to tide me over while I wait to open some better lands.