Well, after giving a try to write a merge scene of a person with an element, I realized that I don't have the enought skills to pull it out. Due that, I decided to withdraw my participation in the contest.
Still, I want to share the plot for the story I had in mind. It's set in the time line the TV serie is current on.
While Aang and the gang are in the Western Air Temple. There, the Iroh (Zuko's uncle) appeared revealing that in order to defeat at the Fire Lord, they must manage the Body Bending.
The Body Bending is a technique that allows a person to fuse with his/her element. To show that, the uncle ordered at Zuko and Aang to launch fire attacks. The uncle absorbed those attacks into his body and transformed into a body made of fire.
After some weeks of practice, Aang, Zuko, Toph and Katara learned to fuse with their elements. Aang body looked like a tornado but with human form. Zuko looked like he was on fire. Toph looked like a statue, but she could move perfectly. And Katara looked the same but in a liquid state.
The four friends attacked at the Fire Lord and Azuka in their element form. They were surprised when both of them used the same technique and acquired a lightning form.
The boys fought against the Fire Lord while the girls fought at Azula. After some attacks, Zuko and Aang managed to weak at the Fire Lord. In despair, he asked Azula's help but she betrayed him killing him.
With her father dead, Azula absorbed his lightning body into her own allowing her to become a small giantess. Her power grew exponentally and the gang was no match for her.
Then, when everything looked to be lost Katara came with a plan making at Aang to recall that he was the Avatar and that he mastered the 4 elements.
Saying that, Katara, Toph and Suko fused with Aang in his avatar state. Azula watched in fear at the form of a giant Aang with his body divided in 4 parts. The lower left part was made of fire while the other one was made of earth. The upper left part of his body was made of water while the other was of air.
Using the 4 elements at once, Aang defeated at Azula forcing her to return to her human form and using his element power, he sealed her in that form so she won't be able to use the Body Bending anymore.
After that, well..., you can imagine how things will go after that
Max