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I had an idea for a poison gym that has all the aesthetics of a grass gym and with all the trainers until you get to the leader use dual type grass/poison pokemon like bulbasaur or Foongus, the gimmick being of poisonous plants.
The whole gym is multiple paths along vines, where your progress is blocked by bulbed flowers and you have to guess the right path to continue. when they open they have trainers in them. Only at the end does the leader reveal the gym's true type being poison, not grass.
The leader's team:
>Quilfish
>Salazzle
>Arbok
>Roserade (ace)
I can't come up with a good badge name, but the leader's whole pre-battle speech is about how it's not always easy to tell when something can be poisonous or not, and as such we often overlook or miss how toxic something could be even when it's signs are right in front of us.
Design wise he'd probably be some sort of toxicology student with a shitty pun name like every other gym leader, maybe Belledon after the nickname for Deadly Nightshade