Well, in 'Avatar The Last Airbender Book 5 Fanfic', there could be the concept of alternate timelines. It could show what would have happened if certain events in the original series had different outcomes. This could lead to really cool scenarios like the Fire Nation being more peaceful earlier or the Water Tribes having a different political structure. Also, the fanfic might explore new cultures inspired by the four elements that we haven't seen in the original show.
One interesting concept could be new bending techniques. Maybe the fanfic explores an advanced form of waterbending that combines with spirit energy. Another concept might be the introduction of new characters who have unique relationships with the existing ones. For example, a new airbender who has a different philosophy about peace compared to Aang.
Some interesting concepts in this fanfic could revolve around the development of the Avatar state. It might delve deeper into how the Avatar can access different levels of power within the state. Additionally, it could explore the idea of non - benders having a more significant role. Perhaps they form a secret society that has an impact on the balance of the four nations. There could also be a new form of bending that is a combination of two or more elements, which would be a really exciting concept to explore.