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.

I had a great time at the Bloomburrow pre-release, winning more games than I lost with a low-curve WR aggro deck. 16 lands, mulliganing aggressively, and practicing when to push and when to hold back were all fun experiences. I enjoyed the set enough that I bought a couple of booster packs afterwards in addition to my prize cards.

As much as I love Boros Valiant as a theme, I didn't quite open up enough of them for a 360 card cube (even with some Theros Heroic creatures and the Agrus Kos Jumpstart 2022 pack to fill in the gaps). I think the theme is close though, so if I end up with more Bloomburrow cards down the road I'll be actively looking for ways to fit it into the cube.

White

Lifegain picked up a couple new creatures in Brightblade Stoat and Lifecreed Duo, while several good Bloomburrow rabbits (Warren Elder, Hop to It, Intrepid Rabbit, and especially Warren Warleader) made go-wide more viable. I added Accorder Paladin in from the maybeboard to supplement this theme. Shrike Force made the cut for being really cool.

Finally, Sonar Strike and Banishing Light rounded out White's removal, which is in a strong place now after the Lord of the Rings commander cards.

Blue

Probably the most important update from Bloomburrow for Blue is the addition of Shoreline Looter. Combined with Looter il-Kor and a smattering of "the ring tempts you" effects and/or Exalted creatures you can now build a decent saboteur deck. I may further augment it with aether figment or Invisible Stalker if 3-4 unblockable 2-drops doesn't seem like too much. 2-mana Blue creatures are a bit thin on the ground in my collection anyway.

Knightfisher, Plumecreed Escort, Splash Portal, and Finch Formation are mostly generically useful effects, but you can build around a bird theme if things line up right - either WU where there are more birds to choose from, or as a subtheme of UG ramp if you pick up Birds of Paradise, Gilded Goose, and possibly Lilysplash Mentor. Knightfisher is mostly here for his stats as an upgrade on Air Elemental, but in order to prevent his rules text from seeming like a trap I added Jeskai Windscout, Aven Fisher, Nephalia Seakite, Murder of Crows, and Dunland Crebain from the maybeboard. Murder of Crows might enable a UB birds variant of a sacrifice deck, although it would probably work better with a repeatable bounce effect. Maybe I’ll end up swapping Splash Portal for Run Away Together.

I may double up on Tolarian Terrors and put Eddymurk Crab back in at some point, but at the moment I’m not convinced that I need two of that effect and I prefer Tolarian Terror’s cleaner implementation.

Dazzling Denial will most likely be played as Quench, but there are enough Blue birds now that I don't think it's a total trap. Shore Up is a utilitarian addition.

Black

Osteomancer Adept is probably the standout addition for Black here, given all the Abzan Food cards from both the prerelease and commander LotR cards. Gurmag Swiftwing came in from the maybeboard as an additional bat for Sonar Strike, and Starscape Cleric is a solid new roleplayer for a lifegain deck. I'm a little less keen on Starlit Soothsayer, but it’s not terrible and my pool of Black creature cards isn't large.

For non-creature spells, Nocturnal Hunger, Fell, and Diresight are welcome if unexciting effects, and with all the Food payoffs running around in the cube Savor should be at least decent.

Red

I would've loved to go full mouse typal here, but I don't quite have the density to pull it off. Manifold Mouse and Brambleguard Captain will have to pull their weight in the Red column without synergistic support. I'd like to swap Lightning Strike for basically anything else, but with only one raccoon making the cut Take Out the Trash isn't taking that spot. I do like Sazacap's Brew over Thrill of Possibility though, and Red picks up a couple of fun combat tricks in War Squeak and Might of the Meek. For some reason I don't have very many Red removal spells in my collection, so Agate Assault is in here too.

Green

Sunshower Druid is fine, and the abundance of food tokens makes Treetop Sentries an easy upgrade on Giant Spider, but overall I was a little underwhelmed by my Green cards from this one. Green did get several nice utility effects in High Stride, Overprotect, Pawpatch Formation, and Longstalk Brawl however.

Gold

Fireglass Mentor and The Infamous Cruelclaw finally give Rakdos something worth playing. Black has the highest density of "the ring tempts you" effects and Red has a nice bloodthirst package, so I'm happy that with these additions a RB deck archetype seems to be coming together. Now Dimir is my only guild with really embarrassing gold cards.

As previously mentioned, Lilysplash Mentor got a Simic slot. I'm not sure you can really build around this yet, as Blue is a little light on ETB effects, but the frog has good stats and there are at least some fun targets for the ability in Green. There's a potential engine here with Ivy Lane Denizen, or the simple addition of trample from Crowned Ceratok. If you wanted to get really silly with it, Bilbo, Retired Burglar triggers "the ring tempts you" on both entering and leaving the battlefield, so a single frog hop will put you at ring stage 3.

Grey

I like little 2-drop guys that filter your mana, and Three Tree Mascot is my favorite one yet. Changeling and more aggressive stats make the Mascot a more interesting card for this job than Shire Scarecrow, especially with the addition of Patchwork Banner. Halfling typal is the most obvious choice to build around, and birds are almost there, but you can also put a decently strong mono-White soldiers deck together with the banner.

Short Bow got the nod for being such a workhorse in my prerelease games, and the Agatha's Soul Cauldron that I opened up in one of my prize packs was flashy enough that I put it in. It's a strong enough card that I'm sure somebody will find something to do with it.