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.
I agree with Xander's post here. They should give a new twist to Poison-types should they make a Poison Legendary.
The Poison-type's offensive capabilities are shallow since it is only super-effective against Grass, but they have good defensive skills, like being resistant to the common Fighting-type. Also, the strategy with Poison-types is to inflict the Poison status condition.
If they do make a Poison Legendary, they should give it a helpful ability. According to TV Tropes, Poison-type Pokemon are often considered the "butt monkey" of all 17 types so if the next generation does an improvement to Poison-types it should give them more love.