This isn't the first theory, and it isn't the last, but I thought I would bring a few interesting facts to the table that lead me to believe that Airbenders still exists. You can add to it or debate on it, just be friendly.

1. In The Winter Solstice: Part 2 Avatar Roku stated that when Sozins comet came to pass and the Fire Nation had won the war, it would be impossible to restore balance. I believe that if Aang was the last Airbender, would that not be damage enough to break balance? Once Aang dies thats it. There are four element, four nations, and four types of people that represent their nations. Without the people, the balance would be messed up.

2. Monk Gyatso is first seen playing Pai Sho, a game in which the Order of White Lotus is based off. The Order of White Lotus is a secret organization that even Iroh is a part of, and they hold many secrets of the world. They long protected Princess Irsa, Zuko's mother. Who is to say that they don't work with and or protect hidden Airbenders? After all, the White Lotus once stated that they discriminate against none. It is possible that Monk Gyatso was a part of the White Lotus, simply by seeing him play Pai Sho hints this, and also he seems like the Airbender version of Iroh. Am I not right?

3. In the episode "Bitter Work" while teaching Zuko how to Firebend, Iroh stated that "Air Nomads are fun loving people." and "Thats probably why they have such a good sense of humor." Lets think about that for a moment. He says it as if he knew or knows an Airbender personally. That would be impossible seeing as Airbenders were supposedly wiped out either before his birth or before he could even talk. He surely can't be referencing Aang, since at the time of this quote he barely ever interacted with Aang.

4. Katara once mentioned in The Sawmp that she didn't know there were Waterbenders anywhere else except for the North and South Pole, yet later in the episode they find a new type of Waterbenders. In The Library and even a few of the next episodes we get to see different types of Earthbenders like Sandbenders. Then in The Firebending Masters we find The Sun Warriors, Firebenders thought to have been extinct for thousands of years. So why then isn't it safe to say that their are other variations of Airbenders out there?

5. Air Nomads LOVED to travel. They were constantly on the go. Sozin only ever mentioned that he destroyed the Air Temples, he did not say that he went to every other settlement looking for Airbenders, I mean common sense would be once word of their temples destruction got to them, they hid. I am almost sure it would be impossible to root out every single Air Nomad there ever was, it's just not likely.

6. When the Fire Nation armies approached the temples, which could literally be seen miles away since the temples were 50 stories above sea level, would it not be possible that the Airbenders hid some men and a lot of women and children somewhere or even put them on a bison and sent them flying away? It is seen in various episodes that their are many hidden rooms in the temples, almost always only accessible by Airbending. How can we not say that some of this doors are hiding Airbenders? Also, it is said that every Air Nomad baby is an Airbender, would that not mean that any pregnant lady who had a baby during or slightly before the attack an un-tatooed Airbender that could very well be shipped off without being known as an Airbender and live to this very day without ever being questioned?

7. In the episode "The Swamp" there was a mysterious tornado that come from no where and crashed the Avatar posse into the swamp. This could have been an Airbender, who wanted to guide Aang to this spot in order for him to learn a few new and important things that will be vital for his victory in the war, and thus help restore Airbenders. Also, along with that the Swamp Tribe has lied unknown of existence for quite a while. Maybe Airbenders have hidden out in the swamp as well.

8. Lastly, but definitely not the least, the title of the name ("Avatar: The Last Airbender") OK, so maybe the name says it all, but lets think this over. In other countries, the show has a completely different name. Some say that its because the term bender is slanderous in European countries, most notably Britain, but I personally don't believe it. If it is true, then why stray so far from the original title? Why not say instead "Avatar: The Last Air Nomad"? Secondly, the title could just be seen as every bodies perception of Aang. Everyone THINKS he is the last Airbender, it doesn't have to be a fact. And just because the title says he is, that doesn't void the show or prevent it from throwing in the fact somewhere later that Airbender still exist.

All in all, I have given my evidence, I supported my theory. I believe Airbenders exist.