» those thousand words ♥
I'd like to say that Magnezone is also OU because of it's typing. Sure, Steel type is kind of a flimsy type depending on what you pair it with, and I'm sure electric isn't the actual -best- of choices, but the fact that it can launch out STAB Thunderbolts and Flash cannons like there's no tomorrow is precisely why it's so likeable. Subs are possible with Zone too iirc, so it can dish out charge beams, slowly increasing Special Attack to devastating levels.
If the Fire/Poison type has this, then I can really see it being either UU or OU, depending on it's stats. The problem with zone is that it can be quite a task getting rid of it's checks/counters, and imagine that going double for this new Fire/Poison Pokemon, but really, I really imagine it being something more like Toxicroak more than anything else. Ground, Flying, 4x Psychic weak. Three weaknesses, and they're all not really all that uncommon, and it's OU. Granted you only really see it in Rain, but even used outside of it it can be a bit of a problem (talking about the Bulk up versions here).
And plus, we really don't know what sixth gen has to offer in terms of boosting status moves. Would it give us something to work around with, as far as weaknesses are concerned? Perhaps there's a status move that allows a Pokemon to negate one of it's weaknesses? These are things I feel that have to be taken into account, because even if a Pokemon type has the most weaknesses in the world, it can still be formidable.
I mean, just ask Celebi. It's Calm Mind does that talking for pretty much every Special Attacker out there, and dishes out devastating Earth Powers, Psyshocks, the whole nine yards, really.