Square Enix's RPG-music hybrid Theatrhythm: Final Fantasy may be taking the stage again. The publisher registered a trademark for Theatrhythm "Curtain Call" on August 27.
The trademark itself (via Joystiq) is fairly vague, covering the usual "computer software" bits and pieces we see in these kinds of trademarks. A curtain call in actual theater parlance is the point at which performers come back on stage to take a bow, which isn't exactly comparable to anything in video games other than rolling credits.
This could be any number of things, from a sequel or special edition to an expansion. Theatrhythm has already provided plenty of downloadable content, both on 3DS and later as a free-to-play, DLC-supported game on iOS.