Metronome / guitar tuner?
But it's already really easy to do this within lsdj. I mean it's great for the crowd that hasn't purchased/doesn't know how to use lsdj; or to have it setup immediately if you use those features a lot (even then, you can just have a dedicated song file for them inside lsdj). A plug n' play program with ps/2 keyboard support kinda like a mGB for ps/2 keyboards would also be a really cool idea, but since it's so easy to set up in lsdj...
+1 for the visuals thing, emulators and sgb's make it worthwhile and not stupid at all.
Also ashimoke, Dauragon and _-_-'s ideas are great.