I imagine monk's "spirit" element to be something close to a third tier element - an element that naturally exists in a different dimension or state of existence, that cannot be naturally perceived by humans. Like, imagine a soul. You're probably envisioning a blue-ish humanoid with no features. Whatever that "soul" is made out of would be the element I would describe spirit as. That being said, I would say spirit-based-powers would largely focus on attacking someone's soul or manipulating your own soul, which would only be possible through some hippie-monk-zen-shit. Nah mean?

Just my $0.02.

EDIT: That would make monks way cooler than just storm, cuz it gives them some next-level shit. I mean, Knights control LIFE and are inherently dope with their near-exclusive role (I mean, as much as you try to say they won't be stereotypical knight-tanks, they will surely be the best to portray such a role, and it is a dope ass role anyway). Monks being able to fight with their souls makes them cool as fuck already, and ascendants are left with the ability to mold the inorganic universe. It's a freaking dope trio: Life/Organic-ness, Spirit-ness, and Physics.