Resonance, if they felt like doing something interesting, could be a "kinetic element" subclass. Think of it this way: each Darkness-themed subclass is thematically opposite a Light subclass element. Stasis "freezes" atoms into a motionless state while Solar agitates them to the point of combustion. Strand is about allowing energy to ebb and flow and using it as it comes and how it wants, while Arc is about taking just enough command of a chaotic energy in order to direct it into the shape and target that you want. Resonance would be the opposite of Void. Void energy is poorly defined, often described as the "closest to Darkness power while still being Light," but it's rooted primarily in gravity-based concepts like singularities. That is, it pulls things towards or condenses them into a central point. Kinetic energy, by contrast, can be defined as stalwart and unmoving. It erupts outwards and builds upon itself in opposition to the idea of Void collapsing in upon itself. Of course, creating a whole new element type to spec into would mean supporting it with new Prismatic aspects/fragments, burns, resist mods, etc so that by itself will likely mean this (and other new subclasses) are very unlikely to happen.