Awhile back Joystiq posted an interview on Guitar Hero: World Tour's Brian Bright, explaining some of the key aspects of the game. One of the questions that came up was, "You allow people to add keyboard sections to their songs. Were you not tempted to create a keyboard controller?"

To this Brian answered, "No, there are reasons why we haven't included a keyboard controller. Not all of our songs have keyboard parts... A keyboard controller deserves to be introduced with its own game. This is something we're thinking about."

Recently, I found another article on Wired, relating back to Joystiq's initial article. It explains how the thought of a "Keyboard Hero" might come to life. Personally, being a pianist myself, I don't think it's a viable game to create. I'd assume that if they did make a piano controller peripheral, it would not be the full 88 keys of an actual piano, and that would suck. I'm sure other pianists would probably complain about it too. I suppose it would be good for beginners, but still, it's probably much better to learn on the real thing/from a trained teacher.

Despite what I just said, I would totally buy Keyboard Hero if it was released.

Source: Joystiq interview: Guitar Hero World Tour's Brian Bright
Source: Activison Ponder Keyboard Hero