Another plotline might involve how the society in Alagaësia reacts to Eragon having two dragons. Traditionally, dragon riders have only one dragon, so this could cause jealousy among other riders or even fear among the general population. It could lead to political unrest and Eragon having to navigate a very complex social situation.
Perhaps in the fanfiction, the two dragons have different personalities. One could be more aggressive and the other more cautious. This causes Eragon to have to balance their natures while training them, leading to a whole new set of challenges and character development for him.
A more adventurous plotline could be Eragon as a dragon getting involved in battles on a much larger scale. He could use his new form to take on powerful enemies that were previously too difficult for him as a human. His dragon form might also attract the attention of other mythical creatures, leading to new alliances or rivalries.
One idea could be that Eragon discovers unique dragon magic only accessible to those raised by dragons. For example, he might be able to communicate with all dragons on a deeper level, not just his own. Another plot could involve him facing challenges from dragon - haters who fear his special bond with the dragons. They might try to break the bond or harm him.
Eragon would probably have a different view on power. Since he was raised by dragons, he would see power as something to be respected and used sparingly, as dragons do. He may also be more fearless, as dragons are often seen as powerful and fear - inspiring creatures. His communication skills with dragons would be far superior, and he might struggle to communicate as effectively with humans.
One interesting plotline could be the rebirth of a powerful dragon with amnesia, having to rediscover its powers and past. Another might involve the dragon's new relationships with other mythical creatures in the fanfiction world.
One interesting plotline could be Daenerys using her dragons to discover a hidden, ancient civilization. The dragons might lead her to a lost city filled with magic and secrets.
One interesting plotline could be Natsu's struggle to fit in with human society after being raised by dragons. He might have different values and ways of doing things that clash with humans.
One interesting plotline could be the discovery of a new type of dragon with unique abilities. For example, a dragon that can change its color to blend into different environments, which adds a new layer of strategy in battles and hiding from enemies.
Well, Eragon is obviously one of the main characters as the title implies he has two dragons. His two dragons would also be important characters. They would have their own identities and characteristics that play a big role in the story.
Another theme is identity. The dragon blood makes Eragon different from others. Fanfictions may focus on how he struggles to come to terms with this unique part of himself, like in stories where he tries to hide his dragon - blood - related traits in a normal human society within Alagaësia.
One plotline could be them uniting different tribes against a common enemy. They use their unique skills, Eragon with his dragon - riding abilities and Nasuada with her leadership and diplomatic skills, to bring peace to the land.