The most off the wall suggestion yet:
For a really out of the box pick, try former mayor and spaghetti western gunslinger Clint Eastwood. There is the little matter of Eastwood's skepticism on the Iraq war, which McCain has pledged his all to win. It's a very unlikely pick, but I'd love to see him debate pretty much any Democrat on national security. "I know what you're thinking. Did Iran really halt nuke development in 2003, or 2005? Or did they stop at all? Do you feel lucky, Barack? Well. Do you?" The squint alone would roast the donks.
Awesome.