Consider (have):
Forest(s) for Swamp(s)?
Scurry Oak - Goes infinite (infinite tokens and infinitely large) with Ivy Lane Denizen, and even without the Denizen it's a free place to put Hapatra counters
Tear Asunder - Removal
Dig Up - Tutor for whatever piece I need to get going, or early game get a land if necessary
* Deathreap Ritual - Might not be reliable enough without a sac outlet, and not really on theme, even though snakes are probably going to die often
* Smell Fear - Proliferate and make a snake fight something
Maelstrom Pulse
* Assassin's Trophy - Cheap, flexible removal
* Plumb the Forbidden - Get value out of a sweeper, or just emergency card draw. Would need more copies
Viridian Corrupter
End-Raze Forerunners
Liliana's Influence + Liliana, Death Wielder: Both are overcosted for what they do, but they do provide some use. Influence is good for getting a big wave of sneks, and can wipe out opposing tokens
Vraska, Swarm's Eminence
Scale Up
Stinging Shot - Can't always be cast, and won't always kill its target, but worst case it can be cycled
Windstorm - Instant speed scalable flier wipe
Silklash Spider - Reusable flier wipe that can block relatively big single fliers
Matsu-Tribe Sniper - Snake that can tap fliers or kill them if they have 1 toughness
* Crushing Vines/Return to the Earth - Destroy a flier or an artifact/enchantment
Orochi Sustainer - Mana dork snake
Defiant Greatmaw
Serrated Biskelion
Pathbreaker Ibex
Viridian Longbow - Decent with infect and deathtouch
Hooded Hydra
Take Down - Destroy one medium flier or all tiny fliers
Skyreaping - Potentially cheap flier wipe, but very situational
Eternal Witness
Spread the Sickness
Sosuke's Summons
Sosuke, Son of Seshiro
Sachi, Daughter of Seshiro
Phyrexian Arena
Journey to Eternity
Blasting Station
Dictate of Erebos
Evolutionary Leap
Fecundity
Explosive Vegetation
Woodfall Primus
Victimize
Lifecrafter's Bestiary
Attrition
Black Market
Meren
Hex Parasite
Cartouche of Ambition - Can easily be 2-for-1ed
Ruthless Sniper - Probably not going to be able to trigger this a lot
Festering Mummy - Probably not worth it without sac outlets
Crocodile of the Crossing, Exemplar of Strength, Quarry Hauler, Ornery Kudu
Grow from the Ashes, Primal Growth, Explosive Vegetation, Skyshroud Claim - More ramp
Crushing Canopy - Destroy a flier or an enchantment
Plummet

Consider (don't have):
Sméagol, Helpful Guide - Make Hapatra much harder to block, and get ramp from snakes dying
$$ Vraska, Betrayal's Sting - Proliferation+card draw, ramp+sac outlet, and a win con all in one
Cultist of the Absolute
$$ Yavimaya
$ Pact of the Serpent - Draw a card for each snake, with extra flavor
$$$$ Finale of Devastation
$$$ Hexdrinker - Cheap snake that can get huge
$$$$ Vivien Reid - Kill a flier, enchantment or artifact, or draw creature(s) and potentially get a great emblem
$$ The Ozolith - Reuse the counters on my stuff to kill opponents' stuff later
$ Harbinger of Night - Gets me lots of snakes every upkeep, but also kills my existing ones so they can't attack
$ Whiptongue Hydra - Wipe away fliers, then stick around to block new ones. +1/+1 counters and a Hydra, though
$ Deity of Scars - Comes with a snake and can indefinitely get counters (combo potential), but otherwise unimpressive
$$$$ Craterhoof Behemoth
$$$ Triumph of the Hordes
Torture
Squall Line - Instant speed scalable flier wipe that also hits players
Gravity Well - Fliers don't fly
Claws of Wirewood - Wipe medium-small fliers or cycle it away
Hurricane - Scalable flier wipe that also hits players
Raking Canopy - Fliers don't fly at me
Downdraft - Bring down fliers for a turn at a time to be blocked by snakes
Aerie Ouphes - Flier removal
Bow of Nylea - Kill small fliers or gain life etc
Crosswinds - Permanently nerf fliers
Needle Storm - Cheap wipe for medium fliers
Pistus Strike - Destroy a flier for cheap, possibly with a poison
Tropical Storm - Scalable flier wipe with potential for extra effect

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Considered cuts:
Ashnod's Altar - I have pretty heavy color requirements
Serrated Arrows - Too slow?
Banewhip Punisher - 4 mana sorcery speed removal, but it does get me a snake
Whispersilk Cloak - 3+2 mana for protection is a lot, but allowing Hapatra to attack for triggers is important
Poison-Tip Archer - 4 mana for a Blood Artist effect that doesn't even get me life is quite a bit, but it has deathtouch and reach, which helps with my weakness to fliers

Urborg, Tomb of Yawgmoth
Westvale Abbey
Scavenger Grounds
Desert of the Glorified + the Indomitable

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Cut lands:
Grim Backwoods
Blighted Woodland

Cuts:
Deathrite Shaman - Targeted graveyard hate is nice, but the ramp is probably too unreliable
Rampant Growth - The worst of the ramp sorceries
Golgari Locket - Artifact ramp is worse than land-based ramp
Song of Freyalise - Can work like a ritual if I have some snakes, with rebound next turn, but the final chapter is very telegraphed and not that great anyway
Puppeteer Clique - Doesn't really synergize with -1/-1 counters just because it has persist. Good card, though
Decree of Pain - Costs a lot of mana, and if I can distribute counters there probably isn't going to be a lot of stuff I need to destroy. Put this back in if I struggle with creature decks
Trigon of Corruption - Didn't realize it costs mana to place the counters! Makes this too mana intensive and slow
Fevered Convulsions - Too expensive to use, probably
Torment of Venom - Too expensive for what it does. Replaced with Incremental Blight
Lockjaw Snapper - Not great if others can kill it to decide when it does its thing. Also, if this does a great job, I'm probably in a good position already
Soulstinger - Has to die to provide enough value for it's cost, which can be hard without sac outlets
Dust Bowl - Don't really have a lot of lands to spare
Pharika - I want the snakes for myself. Can be a good mana sink to get back after a board wipe, though
Path of Discovery - +1/+1 counters can actually ruin some creatures (like Wickerbough Elder). Can help me dig through pretty quickly, though
Harvest Season - It might be hard to tap my dudes and have them survive, but if I get three lands off this it's probably worth it? Wort needs it more!
Noxious Hatchling - Makes a snake when it enters, and can receive -1/-1s with little risk, but doesn't do much else
Sandwurm Convergence - Expensive to cast, and wurms aren't really snakes, but it does force people who want to attack me to brave my deathtouching snakes
Corrosive Mentor - My snakes aren't black. Do I have enough black creatures then?
Throne of the God-Pharaoh – This is mostly only going to be good as a finisher to get the last few points of damage after swinging with a big team
Krosan Grip - Generic removal
Creakwood Liege - Mostly just a lord for my snakes (and other dudes), but otherwise no real synergy with the theme. Also makes me unable to Skullclamp my snakes
Setessan Tactics - Can be a "sweeper" if I have a lot of snakes, but it will cost me a lot as well, and does not work as a sweeper if I DON'T have snakes
Lotus Cobra - I don't actually have any snake synergy outside Seshiro, and not a lot of extra land drops
Skullwinder - I don't actually have any snake synergy outside Seshiro
Sword of Truth and Justice - I need to actually be able to connect with a creature for this to do anything, and then I need to have a few -1/-1 counters out there to get good value
Grim Poppet - Expensive, but lets me double up any targeted counters
Cauldron of Souls - I'll probably have counters already on most my big stuff
Glistening Oil - Slow and too minor of an effect
Phyrexian Hydra - Might not have enough ways to move counters away for this to work
Sol Ring - Ran out of copies and this can't help with the first Hapatra
Seshiro - Only snake synergy, and I don't have a lot of snake cards, so it's mostly for the tokens
Eldrazi Monument - Risky if I don't have Hapatra to produce additional snakes
Decimator Beetle - Just too expensive and slow for what it provides