Poison types are severely underrated.
Their battle prowess as a type is probably one of the worse in the 17 types. Poison attacks are usually reliant on their ability to deter the opponent through Status and stat reductions.
If they wanted to make a Poison Legend, they'd have to give Poison Types a considerable boost in the next Generation. Newer attacks and maybe a few new Abilities that tie into Poison types. Just to put emphasis on the growth.