Hmm… I sort of feel like regular poison is sufficiently underwhelming to consider taking out or reworking completely. It’s strictly inferior to the burn status, and arguably helps the affected Pokémon as much as it hurts in some cases by making it immune to more dangerous conditions like sleep or paralysis. It’s also kind of a slap in the face to the poor Poison-types, who don’t have much else going for them, that most of their attacks are associated with this condition that doesn’t actually help the user much. Just give all poison the Toxic effect. Not sure about what to do with the attacks that currently cause severe poisoning. Maybe have them cause poison as well as something else nasty on top of that, like confusion? Freezing is also sort of pointless the way the game is now – no attack has more than a 10% chance to cause the effect, and when it does come up, it sometimes wears off before even a single turn has passed, having no actual impact on the battle. Why even bother with it?
As for adding something (since I am apparently being required to)… dunno. A lot of games have blindness and deafness as status conditions; blindness would basically just be an accuracy reduction, which is something we have already, but maybe Pokémon could be temporarily deafened by exposure to attacks like Hyper Voice? Deafened Pokémon would be at risk of mishearing their trainer’s instructions and using the wrong attacks until they were recalled to rest. Basically the condition would cause Pokémon to ‘disobey’ (if a Pokémon is already disobedient, well, now it has an excuse…).