I'm going to say that the Poison type deserves a much needed overhaul, only being able to hit grass types for super effective damage and even then only sporadically. The introduction of the Steel type as well as the fact that bug types are no longer weak against them really hurt them bad. I could probably think of a few pokemon types that could be weak to poison that aren't. The Poison and Rock types are the only types to yet have their own dedicated legendary, if I remember correctly, but I think Poison types have gotten it worse considering the rock type has gotten several wonderful gifts over the generations, including that darned gamebreaking Stealth Rock. There are plenty of Poison type pokemon that would be downright great if they made the poison type a genuinely viable attacking option.