I have an idea that could be good for the Guitar Hero franchise: the ability for players to play songs that they had downloaded off the internet.
Basically, players would download the song into their system's memory, then have it transfered directly into the game. As for the button sequences, well, the game would analyze the frequency, amplitude, pitch, etc., and then place the notes at varying spots depending on the music and difficulty level.
If you're still not sold (or never were to begin with), I have one last thing to say: Down the page, in the top slot of my faved music, is a song by weapongod30. Play it, and then ask yourself "What would it be like if I could play this in GH?" If you're still not sold after that, then there's nothing I can do.
[*Edit*]
I forgot to mention: If this idea is indeed used, then it would probably make sense for it to be unlocked by first unlocking all of the game's pre-programmed tracks. That way, the player would at least have a basic mastery of their preferred difficulty before they played anything too hard for them.
New-Milkman
It's been done. Search for "Frets on Fire", it's a PC game that is really close to GH (and is free). It uses an OGG format to make the songs, but I'm not sure how it works (if people place the notes themselves, or the game does it).
phazonfarmer
I know about, FoF. I had this idea at least a week before I'd heard of it. What I was thinking was something to boost their egos by letting them play their favorite songs using something that at least resembled a guitar (as opposed to holding their keyboards upside-down), with an added bonus of letting them show off for their friends on a TV screen (as opposed to them huddling around a computer screen).