Adding a custom offset (a small box where you can type your offset), helps the mobile player a lot in changing the offset
Low detail mode: remove background, remove note hit effect,... (for low-end device)
Loading notice when you waiting for the song to start when starting a song
*Some suggestions that will be cool if you add :
1, custom note color changing, custom UI color
2, add great late and great early so any player can know if they tapping late or early (when playing)