This would be great, especially if generalized to other elements. 5e, unfortunately, has favorite elements (cough cough fire) with a plethora of---often overpowered---spells, while certain other elements are undersupported.
However, I should note that poison is the element that monsters are most often Resistant to, and also often is contested by Constitution Saving Throws which are often monsters' best saves. Wannabe poison users have it really rough in 5e!
