From Gematsu, this week's Jump had a page dedicated to the new game. It looks like a brand new game since it has 200 songs and 60 characters. They are adding songs from newer games (such as FF14 and Lightning Returns), and there's a new versus mode.
The biggest thing I'm taking away from this is that there's a guy who looks a hell of a lot like the guy from Mystic Quest. Which shows that they are going deep with this one. I'm keeping my fingers crossed for someone from FF Tactics Advance. Below is the scan.
The original Theatrhythm was already packed with songs, then they had an additional 52 songs through DLC. On top of that the iOS version added 10+ more characters and 15 or so songs, plus new mixes of songs in the 3DS version (some songs were ended at poor times, like "The Extreme"). This seems to be taking that and adding a lot more, I can't wait for this.
Edit: Just for comparison's sake. 3DS Theatrhythm had 104 songs and 29 characters. The iOS version had 111 songs and 38 characters.