I can't imagine Bonnar fighting Kos
Bonnar is 6'4 and has always been one of the biggest 205 lbers. Kos is 5'10 and although he is thick, he actually has a bone structure that would allow him to do well at 155. Insane size difference between the two IMO but it would definitely be an extremely interesting fight to watch.
However, Kos vs Leben makes a lot more sense. I'd love to see Leben knock that cabbage patch kid out.