Though of course it's not enough to add a weakness here and a resistance there. You have to consider the wider interactions for elements.Poison SE against water and Ice resisting Dragon, thanks. That's all I'm asking. Oh, and a Rapid Spin movetutor
For instance, why isn't grass a better, well-used counter for water?
a) It's offensive coverage is HORRIBLE
b) There's no strong "partner" secondary attack that completes coverage
So the point is even if you give Poison a super-effective hit against water, is that enough that people will actually find Poison a useful attacking type in general?
For instance Electric also hits water for super effective, but in the large scope of things, electric types simply do not pose enough of an offensive threat to make them really effective. This situation might change drastically if, for instance, if Lanturn had 125+ special attack or if Zapdos got Ice Beam. Still, lacking a water resist or particularly intimidating defensive stats make them shaky checks at best.
In Poison's case, Poison is resisted by poison, ground, rock, ghost and steel-- which is really not encouraging . . . The two most notable problematic types are Ghost and Steel, both great defensive presences in the metagame (Rotom-A is everywhere, an Steel is THE king of defense). Steel resists Dark/Ghost, and Ghosts are immune to Fighting and often Ground. Fire comes seems like the best fill-in, but Poison-Fire is laughed at by TTar and every other rock type. Also, you're not hitting Rotom strong enough with fire unless you get STAB-- at which point you're getting screwed by water types again anyway.
See? It's not so simple-- even if you made poison hit water types for super effective, it wouldn't help the situation any more than the current situation of electric types doing jack to check water types.
Trying to use Ice to defensively stop Dragon is riddled with even greater issues (you'd have to COMPLETELY over-hall Ice to make it anything of a decent defensive type-- one more resistance is not going to be enough to get people to run them defensively).
These are just the issues with typing-- it becomes an even worse problem when you consider your odds of getting the base stats and movepools to make decent counters to pokemon as powerful as water and dragon types. :/