There are 3 things I'd love to see in the game... Linkin Park, A setlist with unlikely songs or crude songs, and video game music.....
Linkin Park: Given up, What I've Done, No more Sorrow, Breaking the Habit, In the end, Crawling, Numb, Faint, From the Inside, and more =] I love LP, I think it would be a challenge for singers, yet perhaps on the easy side for most other things. Still, it's my favorite band and I think they would make great songs.
Crude songs: This mostly taken from the Tenacious D movie, I think it would be great and hillarious to have the Beelzeboss song, as well as master exploder and classico or something of that sort. Any type of song from a movie or wherever with that kind of style.
Video game music: I'd love to see quite a few songs in this one as well.... Live and Learn (Sonic adventure 2), Blitzball theme (FFX), Fzero theme, MGS3 Subsistence theme, others I'm too lazy to think of =P
Overall I think each would be quite interesting to play, and it would mix it up from the usual (Widely known rock song) to something more gamers would know.