To be honest I don't understand the need for an Parallax option. Most scripts just need a js-file linked as an atom and a set up via data-attribut.
github.com/mmkjony/enllax.js
I mean that is more than simple in Gantry 5 already.
It's just an effect. And while it may be cool right now – it's just an effect ... but effects are not design. You can already see it get's overused and boring.