John Kerry supposedly asked John McCain multiple times (at least 7) about being his running mate in 2004.
It's not that crazy an idea. McCain was approached by Harry Reid in 2001 about flipping to the Democratic Caucus, McCain himself still had open wounds about how nasty W's primary campaign was, and McCain was somewhat critical of the Bush Tax Cuts on deficit grounds.
What if Kerry-McCain had been the 2004 ticket? Polling of such a thing had them up 15 points over W, but polling before the conventions in 2000 had Gore up similarly so that should be taken with a grain of salt.
What would a Kerry-McCain administration look like?