It's really hard to say. Aang was a very powerful Airbender and the Avatar. He had the advantage of being able to enter the Avatar State more easily due to his spiritual connection. However, Korra was a very physical and aggressive fighter. She had mastered all four elements and was not afraid to take on tough opponents head - on. In a fight, it could depend on the circumstances, like the location and whether they have any outside help.
One interesting plot could be a time - traveling scenario where Aang and Korra meet in a different era. They might have to team up to face a new threat that combines the challenges of both their times. For example, an ancient spirit that was sealed away during Aang's time is accidentally released in Korra's era, and they need to use their unique bending skills and knowledge to reseal it.
It could change the way benders view the Avatar's power. Seeing two Avatars from different times interact might make them realize the vast potential and flexibility of the Avatar spirit.
The theme of friendship and mentorship also pops up a lot. Even though they are from different eras, they can still form a bond. Aang can act as a mentor to Korra, guiding her through spiritual challenges that he has already faced. And Korra can show Aang how the world has changed and how she is adapting to new threats in a different time.
One idea could be a crossover where Ben 10's Omnitrix malfunctions near Avatar Aang's world. Ben and Aang team up to face a new threat that is causing chaos in both their realms. Maybe some of Ben's alien forms have unexpected interactions with the bending abilities in the Avatar world.
In a fanfiction, the Avatar State of Aang could be used in a more creative way. For example, Aang accidentally enters the Avatar State during a cultural festival. His friends are initially worried, but then they realize that in this state, Aang can communicate with the spirits present at the festival in a unique way. He learns secrets about the spirits and their connection to the human world, which he then uses to strengthen the bond between the two realms when he's out of the Avatar State.
In a 'Korra meets Aang' fanfiction, a storyline could be that Korra is having trouble with her connection to the past Avatars. Aang then comes to her as a sort of mentor, guiding her through the process of strengthening that connection. They could also face a common enemy that is trying to sever the Avatar's connection to the past altogether, which would force them to fight side by side and use their combined powers and knowledge to overcome the threat.
If Korra meets Aang in fanfiction, they may have a deep conversation about the role of the Avatar. Aang could share his story of how he had to end the Hundred Years War and the toll it took on him. Korra could then talk about her struggles in the modern world with things like industrialization affecting the balance of the elements. This exchange would be really interesting as they compare and contrast their Avatar journeys.
I'm not aware of any well - written 'lemon' fanfictions for Korra and Aang as such content often goes against the family - friendly nature of the Avatar series. But if you're looking for well - written fanfictions about them in general, there are many that explore their relationship in a more appropriate way, like their roles as Avatars and how they could interact in different scenarios.
One great aang and korra fanfiction is 'The Avatar's Bond'. It beautifully explores the relationship between Aang and Korra across different timelines. They face challenges together and learn from each other's experiences as Avatars.