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The TYPE doesn't need a fix all that badly but perhaps it would enjoy some buffs
>remove Ground resistance to Poison; toxins leach into soil just as well as the soil dispels them
>Let Poison-type Pokémon be vunerable to being poisoned, but let all Poison-type 'mons have an inherent Poison Heal (might be best balanced if the poisoning cures on switch and/or by disallowing Poison-types to be poisoned by entry hazards)
>Poison Stealth Rocks- Tox Rocks- which act like a Toxic Spikes that hits non-grounded Pokémon
As for individual 'mons...
>give Nidoking Gunk Shot and Swords Dance; it's pretty fine as is though
>give Weezing Slack Off
>Swords Dance for Arbok and Seviper? though Seviper doesn't really get much of a chance to set up, huh; also buff some of Arbok's mediocre as fuck stats
>Regenerator and Gooey for Grimer and Muk, and maybe Garbodor too
>Recover for Gulpin and Swalot; maybe give them a cool ability like Thick Fat?
>Give Trubbish and maybe Garbodor Scrappy, Wonder Skin
>Iron Fist, Rough Skin, Huge Power for Garbodor? also please buff Attack 5 or even more points so it can have a solid 100+ Attack stat
>Skuntank might appreciate Slack Off, Swords Dance, and/or Gunk Shot?
>buff Drapion to give it at least 100 Attack and 100 Speed; maybe Sucker Punch, Gunk Shot too, though that might honestly be a little frightening
>Give Toxicroak Water Absorb over Dry Skin; maybe Prankster too?
>Give Recover to Dragalge, maybe?
>maybe buff Vileplume's defensive stats a little? maybe give it Chlorophyll too?
>Gunk Shot and Compound Eyes for Ariados and just completely retcon that abysmal Speed stat entirely; give that poor fucker a Speed stat of 80 or 90 or even 100, WHY IS MY BABY SO SLOW GOD IT HURTS TO SEE
>Recover, Iron Barbs, Gunk Shot for Qwilfish
>unlisted 'mon are either hopeless or fine as they are
>pic related is the former and I cry every night
>TL;DR: give it Gunk Shot and/or recovery