Preliminary npcs:

Green ring: Wolverine (he was willing to kill the woman he loved to save the Earth, and drug his upper body for several miles after Hulk ripped him in half.  That's some insane Willpower)
Orange ring: Dr. Doom (he craves power above all else)
Red ring: Dr. Banner (who appears in the [url=http://greenlantern.wikia.com/wiki/Blood_Ocean]Blood Ocean[/url] when he's sent to the DC universe, otherwise, Thanos and half the galaxy would have to team up with us to fight him.)
Yellow ring: Kilgrave

Anyone have any suggestions for the other rings, or reasons these don't work for these characters?