If we were to ask Activision what the most-requested feature for Guitar Hero is, I could almost bet it would be user-generated content. By that, of course I mean putting in your own music and creating charts for it. While Harmonix has expressed an interest in integrating such a feature in Rock Band down the road, we really haven't heard Activision's stance, at least not until now.
Bobby Kotick of Activision touched briefly on the subject at the recent Morgan Stanley Technology Conference. He noted that it is a huge deal for them to get that into the game, but said that he doesn't see it happening in the next 5 years. Don't worry though, they plan on making it happen, just not very soon.
I can imagine that the hold-up isn't creating the tools that are necessary for gamers to create the content. Rather, I think the problem is that they are going to have issues preventing people from uploading copywrited songs. Once people can do that with ease, there won't be any need to buy further Guitar Hero games, which is kind of bad for business.