I'll admit, I had some trouble with this one at first because it was difficult thinking of a property that I enjoy that didn't have a licensed game associated with it already. Thankfully, the right answer for myself hit me lick a sack of bricks while I was looking through the DVDs on my shelf yesterday, trying to find a movie to watch to kill some time before heading down to the game store.
Without a doubt, I would love to see an official Avatar: The Last Airbender roleplaying game. I'd love a game that gives me a chance to run a campaign within the world of either the original series or The Legend of Korra's version of the setting (which is the series I personally prefer).
Now, I guess we have to ask ourselves one question: What would the Avatar: The Last Airbender Roleplaying Game look like? Well, I assume it would either use a classless system where players could build their characters exactly the way they wanted. I also see it being on the lighter side of the mechanical spectrum, leaning towards the narrative side of things. I also imagine the game using a free-form bending system to allow the players to be creative with their elemental abilities.
I'm actually kind of surprised nobody has approached Nickelodean about doing this already. I know I would pick it up in a heartbeat, and I would be surprised if I was the only one.