Rainbow Rocks figures are now listed on the Equestria Girls website, or at least Rainbow Dash and Pinkie Pie. Chances are we will see a lot of characters that are missing from the first few waves in this one, including possible sirens.
Unfortunately they don't have much else to go off of. They are listed with not much else over on the German site. Hopefully more information is revealed soon!
Thanks to Twilight Sparkle for the heads up!