It's for the best that the Guitar Hero game was canceled. I'll give credit for not wanting to make another Rock Band knockoff, but the six-string sounds like an awkward middle ground between the accessible 5-button gameplay and the authentic real guitar gameplay, and lack of compatibility with existing controllers and songs wouldn't have gone over well at all.
Yeah. Even though I'm a plastic-instrument junkie who owns almost all the GH titles in addition to everything Rock Band, stepping away from the co-op element of gameplay would have made it a non-starter for me.
As much as I'd love an RB4 or GH7, I'm glad this road never happened.