Poison lowers all attack power and all defences, but it doesn't do anything with resistances. I say why not? There are two ways to go about it:
Let' assume, as my science indicated, that minor poison reduces defence by 15%, moderate by 17% and strong by 20%. If you're wearing full Vog for 8 points of resistance, minor poison would reduce that to 6.8, strong would reduce it to 6.48. If you had 0 resistance, it would simply not be affected. Negative resistance would be boosted by 15% to 20% (Probably more if you also have negative poison.)
Poison itself would not get reduced, to avoid recursive shenanigans.
Another way to do it is to have minor poison reduce resistances by 1.5 bars, strong by 1.9 and then just scale it up or down from there depending on poison strength and resistance.
In practice, this'd make poison more of a threat in levels where poison and other statuses mix (C42, poison levels containing rocket puppies and mecha knights), it'd make poison a valid option for Lockdown (as it could bring skolvers and shockwalkers just under immunity), and it would, perhaps, allow you to set a fire-themed enemy aflame by the use of strong fire or something such. There are also enemies with partial resistances to statuses, such as most slimes vs stun and Darkfire Vanaduke.
Perhaps with the introduction of this, Vanaduke would be immune to minor freeze, with the possibility of poisoning him below it.
Or maybe we could make it impossible to waltz through haze bombs without 9 resistance. I'd prefer both, though.