I'd say that between the two of them, Arcane and Primal have already covered all the obvious flavor ground of an elemental power source, hitting both the "control the elements" and "call upon elemental spirits" tropes. In order for an Elemental power source to not feel like a rehash, it'll need to have a distinctive mechanical difference, like Psionics with Power Points and Augmentable At-Will Powers. Given that it's looking quite likely that Shadow will not be fleshed out into a full set of classes, but rather treated as a mix-in that provides new options to existing classes, I'm currently quite doubtful that they're even attempting to create a set of Elemental classes, though maybe we'll eventually see a set of Elemental-flavored builds for existing classes.