Visual Groove Editor
It would look much like a 16-band equalizer, except that instead of frequency bands it would control the timing of each 16-th note of a bar. A negative value means the note is pulled ahead of time, a positive value means it is pushed back. This way you can draw your groove with the mouse, or just take a look at the groove you extracted from a clip.
A similar view would be needed for the velocities of the notes (which are also part of the groove).