During Practice/Play Mode, add a part re-do feature on pause wheel that allows you to re-do a certain part of the song (e.g. Part 3 of 4) that you might've performed badly.
Saves re-playing the whole song?
Andrew Leighton commented
You can scroll the brackets at the bottom of the page to restrict the replay to a specific region of the song you want to practice.
can't you do precisely this when you select the song, and it shows the stars you've got before from each part of the song and just click the part 3 for instance to play that alone.
However, you wont get 5x score immediately so you wont get world rank 1 scores by playing individual sections, save the first one per song.