Kos would be a good match up for Diaz because he wins it pretty dominantly.
The Maia fight to me would probably go all Nick's way on the feet, but people would then say Maia faught the wrong fight or make excuses like they did with BJ.
Fitch would be a hard hard fight for Diaz. It's the complete style he would struggle against (to be fair, it's GSP's style too). Would be a true test to his TDD and BJJ.