No. The 'Eragon' series has a large following for its story of adventure, magic, and friendship. Adding sexual content through fanfic is not in line with the general tone and values of the fandom. It can also make many fans uncomfortable.
In the official Eragon lore, we have epic battles, the bond between dragon and rider, and the exploration of a richly detailed fantasy world. But any form of sex stories are completely absent. The series is aimed at a wide range of audiences, including young readers, so it adheres to a certain standard of propriety.
To my knowledge, there isn't a 7th book yet. But you never know, the author could surprise us in the future. Follow their social media or literary blogs for updates.
Saphira is often a popular character. Since she is Eragon's dragon, she can add a lot of depth to the story. Her unique abilities and her relationship with Eragon can influence the interactions between Eragon and Murtagh in the fanfiction.
The OC could be integrated into the story by having a shared past with either Eragon or Murtagh. Maybe they grew up in the same village as Eragon but were separated when the Ra'zac attacked. Years later, they reunite and the OC can offer some unique perspective or ability to the situation. Or, if the OC has a connection to the dragons in a different way than Eragon and Murtagh. For instance, the OC might have a special bond with a dragon that was thought to be lost. This would bring them into the story as they try to protect their dragon friend and get involved in the events related to Eragon and Murtagh. Also, the OC could be a trader who has information about the movements of the enemy forces. They could trade this information to Eragon and Murtagh in exchange for protection or some other favor, thus becoming part of the story.