i can think of a number i'd like to see:
Beatles - would have been fun to see early in the career with all those screaming nutbar fans, but i prefer actually hearing
Doors - of course just to see Jim in all his drug haze glory
The Sex Pistols - early club days when punk was just breaking, never was or is a such a groundbreaking definitive few months in music. Changed everything forever.
The Clash - see above
Nirvana - it would have to be early Seattle days, just before or after Nevermind....to get a feeling of that band that was just about to blow the music world away
Bright Eyes - i get the feeling that i may never have a chance to see him in Saskatoon. An all acoustic set would be amazing
Pearl Jam - can't see these guys too many times, they seem to be getting better with age, though would have been fun to see them just as they were breaking with Ten
Others - Fiery Furnaces, Arcade Fire (sold out before i could get tickets
), Apples in Stereo, Supergrass, White Stripes, Radiohead, Spoon, New Pornographers, etc, etc. I'd better stop, could think of a million.