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.
Obviously, Eragon himself is a main character, now in his dragon form. His dragon Saphira could also be a very important character. She might play a role in helping Eragon adjust to his new form, teaching him things about being a dragon that he doesn't know.
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.
There could be many. One might be about a new adventure of Eragon and his dragon where they encounter a mysterious lemon - scented forest that has magical properties. It could involve them using the unique elements of this forest to overcome a new threat.
Well, in some 'Draco turned into a ferret fanfiction', there could be a plot where Draco in ferret form discovers new things about the Hogwarts grounds that he never noticed as a human. Maybe he stumbles upon a secret passage or a hidden magical creature. Also, there could be a story where his transformation is a result of a misfired spell by a student, and the whole school has to figure out how to reverse it.
One possible plotline could be Naruto having to adapt to his new fox form and learning how to use its unique abilities. For example, he might discover that in fox form he has enhanced speed and stealth, and he has to figure out how to incorporate these into his fighting style.
Another plotline could involve Sookie using her new vampire powers to solve mysteries in the supernatural world. Since she already had some unique abilities as a human, becoming a vampire could enhance them and make her a sort of detective in the vampire community.
Another plotline could involve Megatron using his knowledge from being a Transformer in his new human form. Maybe he uses his strategic skills to solve human problems, like helping in a corporate takeover in a more ethical way than the humans were planning.
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.
One interesting plotline could be Natsu trying to adjust to the new physical form while still having the same adventurous spirit. For example, Natsu might initially struggle with wearing different types of clothes that are more suitable for a female body. Another plotline could be how Natsu's friends react. Some friends might be shocked at first, but then quickly accept and help Natsu through this strange transformation. There could also be a plotline where Natsu discovers new abilities or sensitivities that come with being in a female form.
One interesting plotline could be Bella's struggle to control her new vampire instincts while still maintaining her relationships with her human friends and family. For example, she might have a hard time being around them without revealing her new nature.