If you want a game that realistic why not actually play a six string guitar?
In short, it makes the learning process easier from the start, and less boring for younger ones who might not have the patience and stick-to-it needed to master a normal guitar.
I'm not sure what the interface will look like, but imagine the possibilities of displaying tablature along with musical notation along with what notes you're actually hitting, perhaps even with a photo of the desired hand position for a chord, all simultaneously (or easily accessible).
Then, add training modes, (like guitar hero but with a full metronome range), technique exercises for scales/arpeggios, etc... and you have a powerful training system.