Hardened Scales is a card that powers an archetype that is powerful in Modern and looks like it could make the jump to the Pioneer format. We're missing the crazy fast combo potential of Arcbound Ravager, but there still seem to be some powerful options to work with.
Hardened Scales Decklist (Pioneer)
4 Hardened Scales 4 Winding Constrictor 4 Hangarback Walker 4 Walking Ballista 4 Steel Overseer 4 Stonecoil Serpent 4 Once Upon a Time 4 Thoughtseize 2 Animation Module 1 Evolution Sage 1 Metallic Mimic 4 Blooming Marsh 4 Llanowar Wastes 4 Overgrown Tomb 2 Woodland Cemetery 4 Swamp (339) 4 Forest (347) 2 Karn's Bastion
We may be lacking some of the combo power of Modern, but we can still go hard on the Hardened Scales theme by including four more copies in the form of Winding Constrictor. One of the feel bad moments in the Modern version of this deck is when you cast Ancient Stirrings and see your copies of Hardened Scales go to the bottom of the deck without being able to take them. The Pioneer version runs Once Upon a Time as its card selection spell of choice, so we'll have access to a ton of our main effect.
Once Upon a Time is pretty much an automatic inclusion in any deck that runs green and creatures, so it's obviously great here. Our various creatures play wildly differing roles, so we can always just select the one that is best for our draw or the current matchup. It also goes a long way towards smoothing out our mana, so it's possible we could shave a land or two. OUAT also helps us find any singleton cards or sideboard creatures that we may run a higher percentage of the time.
This shell of artifact creatures that use lots of +1/+1 counters directly ports from the Modern version of this deck. Walking Ballista is notably quite good in this format, even without knowing whether people will be running small creatures, as one of the most feared decks is the Felidar Guardian / Saheeli Rai combo. Even a single counter on a Walking Ballista is enough to stop this combo, as you can simply ping the Saheeli if the try to go for the copy effect. Hangarback Walker, Steel Overseer, and Stonecoil Serpent let us play an aggro game that gets out of hand when we have our key enchantment in play.
It's easy enough for us to include black mana to support Winding Constrictor, so we also gain access to one of the strongest cards in the format in Thoughtseize. We're expecting lots of combo decks to be played in the early days that include discovery of which cards deserve to be banned in Pioneer, so Thoughtseize gives our (relatively) fair deck a fighting chance at making the game go long enough for us to win.
Animation Module and Metallic Mimic are both cards that work well in this strategy and with each other, but we've trimmed the numbers on them for our initial build. Evolution Sage is a speculative card to try, though it's awkward that it loses access to fetchlands and that Fabled Passage is not a great fit in this deck.
Our colored mana requirements are not very extreme, so we can certainly support some utility lands. Karn's Bastion is perhaps the strongest option available, though we could also try any of the creature-land variants or Ruins of Oran-Rief.
We're used to thinking of Hardened Scales as an artifact-based aggro deck with a key card that happens to be green, but since we've already lost access to Arcbound Ravager, the only real artifact synergy left is Steel Overseer. Perhaps we can build a version of the deck that focuses more on actual green creatures. This goes a bit deeper into unexplored territory, so let's start by taking a look at a few of the creature packages we could consider.
Pelt Collector and Experiment One are very potent 1-drops that use +1/+1 counters very well. We just need to find a reasonable curve of creatures that could actually grow them more than once. If we find a good Evolve curve, we could also add blue for Cloudfin Raptor.
We'd have to go deeper to fully support the graveyard theme, but Lotleth Troll with Slitherhead is a very cheap way to create a lot of trample damage for zero mana.
Nissa, Voice of Zendikar is probably worth considering in these lists, as it provides an immediate pump effect or extra bodies, whichever is needed. Renegade Krasis and Drana, Liberator of Malakir are very synergistic with the strategy but are likely too slow.
Avatar of the Resolute and Barkhide Troll would be strong cards in a Green Stompy version of this archetype. It remains to be seen if the format will be slow enough to allow this deck to exist.
There's no shortage of creatures that can gain +1/+1 counters repeatedly, so we're excited to brew more within this space!