A relatively clean solution to weave the ability tree and poison would be to make vulnerability multipliers like Curse powder special and Eldritch Call affect the damage that poison deals.
Since curse is no longer a +210% mult for poison, I think this is relatively reasonable to incentivize players to put in some manual work without nerfing (or buffing) the current "afk" type gameplay that poison provides. It's also thematic with the idea of a hexes/curses/poison/ailment archetype. Realistically in raids, someone running poison should be using curse on boss to help out more, this just incentivizes it for the individual.