Russell, you can write any story you want, so long as you don't infringe on any trademarks you're fine legally (but that is not legal advice and I am not a lawyer).
e.g., Last Son of Tomorrow by Greg van Eekhout:
http://www.tor.com/stories/2009/05/last-son-of-tomorrow
(which I really mis-like for the whole justified social-opposite-reaction thing --- people who believe in moral relativism have never had to confront evil on the back of a school bus --- no one is responsible for a person's actions but that person and nothing justifies doing harm to an innocent)
William