I've been playing around with an acid fiends druid and it's been very fun.
After investing in enough -psn res and using vine companion they pack quite a punch against normal mobs when their poison flash triggers. This doesn't happen too often though.
I think it might be fun to have an SU Druid Helm that substantially raises their chance to cast poison flash on attack. Or it might just dramatically increase their attack speed, which would essentially have the same effect.
A lot of SUs are centered around improving a single skill and I think this addition would make the spec even more fun. This might turn the spec into an okay/goodish farmer and that's really all I would want from this skill.
Edit: I don't think this would make the skill OP, since you also need to max it's synergies (effectively needing to max 3 skills) to make it work well and the skill level of the poison flash is fixed.