Okay, there's a lot of music apps for the iPad, but very few set up for live band performances. So here's your idea, go ahead and use it. If you make it a pais app I only ask for a free copy for a friend.
There is a need for an app for bands that perform live music. There are some that work as music controllers and the like, mostly a lot of home rolled apps, but I'm talking about something else. What would be helpful is if a band could set up their playlists with lyrics and maybe tabulatures or notes all tied to the individual song. Then save them as cards, say (just an organizational idea). Then for their live performances the ability to juggle the songs as playlists, doing a preset for the order of songs with the ability to change on the fly (so, read cards in order, but can quickly flip between all cards).
That's the basic idea. If you could tie in things like, say, a tuning app, maybe a music washing section (like adding effects to a local channel), a sound board app (that could be it's own app, a commercial release would be cool), maybe a sound visualizer might be helpful.
Anyway, the song/playlist app would be th emain thing. Go forth and make your millions.