AHHH! I'd pay to see King Diamond vs. Barney...actually, I'd pay to see King Diamond vs. just about anyone, just because it'd be so weird. Maybe he would defeat his opponent by letting loose with an ear-piercing wail and the other competitor's head would explode, like in Scanners. Or something.

How about Kraftwerk vs. all of those Miami Bass guys who sampled all of their records?

Pan Sonic vs. Suicide?
The Scissor Girls vs. The Sun City Girls?
Yngwie Malmsteen vs. everyone in Guitar Center at any given time