It would be pretty neat to have an ability that let Poison types damage Steel types. Heck, considering how bad offensive Poison is even without Steel's immunity, said abilty should make Poison SE Steel. Call it Corrosion or something.
Poison-type moves that specifically affect Steel-types would be neat, too. Something like
Acid Wash
Special Poison
80 Power
100 Accuracy
15 PP
Deals SE damage to Steel types. 20% Burn.
would be pretty neat.
For some other possible game mechanics, I once had the idea that there should be a poison-type weather condition called Acid Rain. It would deal 1/16 to non-poison-types, 1/8 to steel- and grass-types, and maybe a 25% boost to Poison-type moves' power. Another idea was an new status condition called Chemical Burn, which would be like normal burn, but reducing special attack. Obviously, moves with acidic effects would inflict said condition.
That all said, I still think Poison SE Water would go a long way toward balancing both offensive Poison and defensive Water.