The water-cooled rotor has already been done, forgot where I saw it though.
Another neat thing that's been done on mountian bikes (prototype only) was a twin-rotor disc brake. It used one 4-piston caliper, with 3 pads. One pad on each side and one in between the rotors, pretty neato. The caliper was water-cooled, too. Trying to find a pic but it's tough.. Never went into production because it was just too powerful.
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1988 Quad 4 Coupe 5 speed click for more pics
1986 GT 5 speed
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