What i am about to suggest may have already been suggested before, but here goes:
* = Inspired by DDR.
MAJOR:
* Versus mode (on one computer).
* Double mode
* (Maybe) unison and a battle mode
* A more balanced health gain / health loss ratio
* A background that changes during the song.
* An announcer.
An option to change the controls.
* Multiple difficulty levels for each song (Light, Standard, Heavy, etc.).
* More ratings for each step eg. {Marvellous, Perfect, Great, Good, Poor, Miss} as well as "boo" for pressing too much.
* Freeze steps.
* Some songs that come from DDR. (With permission of course.)
MINOR:
* A preview of a selected song before it loads.
* An option to change the speed of the arrows (the current setting of about 2x DDR is a bit much at times...)
* An option to turn off 'flat' mode (All arrows that are 'on beat' in one colour, and in between in another.)
* A health bar and score indicator that animates, changes colour etc.
* Animating arrows (including the ones at the top)
* A more accurate estimate of your score during the song.
* A grading system.
A bigger variety of character emotions (and more accurate too).
* Something more attractive than brackets to indicate difficulty.
A max combo statistic when the song is picked.
A more reliable high score system.