I'd like to see that Kos VS Diego. But as a business standpoint, I don't see a good "business" reason the UFC would have this fight now. They have two different guys in them, to challenge for the WW title. If one defeats the other and goes on to be defeated during his title shot, they both lose credibility to fight that champion.
One title fight= $$$
Two title fights= $$$$$$
UFC is a business guys, and as much as we hope and wish for match-ups, and despise others, the UFC isn't going to thin out it's top-tier of fighters be having them fight eachother. Sucks but thats how it is....