Red

I like the card advantage that Anep, Vizier of Hazoret gives, plus the fact that it has trample over Scytheclaw Raptor's ability.

It's amazing how far we've come from Jackal Pup to Greasewrench Goblin.

Shock Brigade generates a token / sac trigger every turn and is great at carrying equipment.

Irreverant Gremlin is a card that everyone else seems to play, so I figured I'd try it out. Skittering Precursor is just okay.

Harried Dronesmith is fine, but red wants another big drop at 4, so Bloodfray Giant comes back in.

The fact that Cacophony Scamp can hit creatures makes it much better then Heartfire Hero, although this is another instance of having to remove some amazing artwork from the cube.

Green

[[Evolution Witness] is good once it gets going, but super slow. Sutina, Speaker of the Tajuru is Civic Wayfinder on crack, plus it opens up a little counters / landfall synergy.

Altanak, the Thrice-Called has a way to get itself into the graveyard, plus is bigger and easier to cast than Plated Crusher. It dies to removal, but does draw you a card in the process.

Migrating Ketradon does a good enough Generous Ent impression that it is welcome in over Great Oak Guardian, which i found never really generated the kind of combat blowout you think it could.

Being able to cast Monstrous Emergence before you have the ability to deploy a big creature makes it the best Rabid Bite effect IMO.

Greenbelt Guardian is an Untamed Kavu that you never have to commit to a mode on. Beastbond Outcaster has not impressed.

Streetwise Negotiator is a generically good card that does a bit of counters enabling, but Hydra Trainer additionally provides a nice payoff.

Similarly, Aloe Alchemist is generically good but I find the repeated "explore but no counter" trigger on Traveling Botanist to be very interesting.

Repeated exploring lets us cut Merfolk Branchwalker for Splinterfright, a fantastic card for green graveyard decks.

Aftermath Analyst is another, doing a bit of self mill, blocking small creatures well, and then giving value mid-late game.

I don't see Berserk played that often, but Synchronized Charge is another good counters enabler.

Elvish Regrower has 4+ power and graveyard recursion on ETB, what's not to love?

Gold

Reflector Mage has drawn a lot of complaints of being miserable to play against lately, so I'm swapping it out for Soulherder. I think a lot of complaints about the latter were due to its interactions with the former, but will still keep a close eye on it.

UG tends to be more of a tap out big mana deck, so Skyserpent Seeker seems more on theme than Mystic Snake.

Host of the Hereafter starts with counters on it and has a better payoff than Expanding Ooze.

Speaking of counters, BW has an emergent counters theme after intentionally building up GW and GB, so I'm putting in Ondu Knotmaster and Bartolomé del Presidio to solidify that.

Hybrid

I think the TDM cycles of 3-color two-brid mana cards are really interesting when you consider that they are basically 3 different 2-color cards, so I'm going to try some of them out. I really wish I could play Reputable Merchant instead of Kin-Tree Severance, because it makes so much more sense, but the body on it is just so bad for 4 mana.