Maybe this is the replacement for Narnham Renegade you're looking for? It certainly fits both the resilience and +1/+1 counter theme, though it might be too unexciting if green is not in on the sacrifice theme.
Also, you have red as an aggro deck in two color combinations, maybe if it feels like a trap you actually need to add some more aggressive tools that lend themselves well to the BR and the RG color combinations? Or, alternatively, aggro shouldn't be an archetype in those color combinations?
I would also look at your typical spells matter deck. Is it a slow deck or a fast deck? From the low number of counterspells, flipping a Delver and riding it to victory seems like it's not supposed to work. Maybe you can replace it with the spells matter card that has been most sastisfying in my cube?
As for Pack Rat, if you're players are not discarding every card in their hand to make more rats (at the end of their opponent's turn to avoid wraths), they're doing it wrong. It's hard for me to view this card as anything else than game ruining bullshit, and I can't think of a format I would want to run where this is philosophically the right card for the job. At the end of the day, Pack Rat is simply a removal check. If you have it, great, if you don't, well shit, you are going to face a million million/million rats soon. Good luck with that. I know it's not unbeatable, but there's also a reason Pack Rat was a single card archetype in Return to Ravnica limited that was practically unbeatable if you drew the card.
I did have a Young Wolf sitting in my maybe box for a couple months, that's a reasonable pick. Green has some tools for the sacrifice deck, I tried to bleed most of the most prominent archetypes (Aristocrats, Counters, Tokens) into every color to some degree that way there was more flexibility with what enablers were available for those primary archetypes (which has thus far worked fairly well, and if I had better fixing would probably inspire a lot of 3-color decks).
In my head there should be enough support for Red to be aggressive for the early game and curve out into a good midrange deck with strong threats to close things out, but I think people are looking for a more traditional red deck that stays very low to the ground (which honestly most of the cube is low to the ground) and then finishes with burn, when most of the burn in this cube is creature targeted (intentionally). Red should play as more of a tempo/midrange deck, I may need to listen to you and Sigh, and either invest a bit more or cut it as an archetype, I feel like RG Aggro is a viable deck, I've seen it, and lost to it with otherwise good decks, but it really is looking to finish the game with creatures or things in the category of Berserk or Ghor Clan Rampager, and less with a flurry of Searing Spears and Lightning Bolts.
I love Rise from the Tides, it's a great reward for the spells deck and I could easily see it taking Delvers place, I might need to take a look and smooth out the curve, but I could be very happy with that.
and I can see cutting Pack Rat, it's either game-winning or just really poor.
If I might ask, what is the primary color in Aggro supposed to be? Kinda seems like maybe
or
? Don't really see many clear signals into a "traditional" agro deck throughout the cube. That's also fine, but if this is your desire, I'd be very careful to remove undue hints throughout all the colors. In general, I'd be cautious of what exactly you consider "quintessential" if you are set on building a unique experience.
I would say that Green is the primary aggressive color, with black and red being good supporting secondaries. But you might be right, Aggro might just not be a strong archetype in the cube, most of the decks are very low to the ground in general, and I think the layout of the cube favors a tempo or a midrange style deck. I'm OK with that, again, in a lot of ways I'm trying to make this more of a feel-good cube, and I think losing to Aggro can be frustrating experience since it (ideally) provides you very little opportunity to react or change your course to stop them (especially in limited where you just might genuinely not have the tools to do so). I'll have to sit on it and think about what I should and should not be prioritizing.
As for cards that I consider "quintessential" or just very compelling/draft format defining for their time/color I'd say...
: Mother of Runes, Thalia, Guardian of Thraben (this one is probably just personal bias), Fiend Hunter, Restoration Angel, Cloudgoat Ranger, Swords to Plowshares, Wrath of God, Pacifism, O-Ring
: Delver of Secrets (this is probably more indicative of my favorite draft format at this point), Frost Lynx, Mist Raven, Brainstorm, Daze, Gush, Binding Grasp (Just in that it is a mind control, not this card specifically)
: Carrion Feeder/Gravecrawler (again, probably a personal thing, but it tends to find its way into any cube I make), Pack Rat (again, this is probably more warping or damning than it's worth), Desecration Demon (just in having a 4 drop big demon with condition), Vendetta, Dismember, Animate Dead, Stab Wound.
: Monastery Swiftspear, Flametongue Kavu, Purphuros, God of the Forge, Kiki-Jiki, Mirror Breaker, Lightning Bolt, Faithless Looting, Madcap Skills.
: Birds of Paradise, Experiment One, Eternal Witness, Reclamation Sage, Berserk, (Assorted Fight Cards), Green Sun's Zenith, Rancor, Birthing Pod (this one really probably SHOULD be cut, I've genuinely never seen it played)
These are the cards that when you say "Cube" they jump into my head as most likely auto includes that should have some amount of support. There are others that I have at this point cut. Not sure if this preconception adds more or takes away more from the cube.