Perhaps a plotline where the returning humans bring new technologies or ideas that either clash or blend well with the existing magic in Ooo. They could introduce things like new forms of communication or transportation that change the dynamics of different kingdoms. Maybe they start trading their technology for magic items, creating new economic and social relationships.
There could be a fanfiction where a group of humans from our world accidentally stumble into the Adventure Time universe. They then have to deal with the magical and often dangerous elements while trying to find a way to return. Along the way, they interact with various characters and learn about the unique culture of Ooo.
A great plotline could involve Jake as a female and her unique take on being Finn's best friend. She could bring a different kind of energy to their adventures, like being more nurturing while still having that tough side when they face villains.
There could also be new human characters created specifically for the fanfiction. These could include leaders among the humans, or characters with unique skills or personalities. For example, a human scientist who is eager to explore the magic of Ooo, or a young human who befriends the magical creatures right away and becomes an ambassador of sorts between the two groups.
One interesting plotline could be the return of a long - lost Avenger who has been in hiding or captivity. For example, they might come back with new powers or a different perspective, causing a shift in the team dynamics.
Another plotline might involve Merlin coming back to find Camelot in chaos. His return then becomes a journey of restoring order, not just through his magic but also by reuniting the people and rebuilding their trust in the kingdom. He might have to deal with those who have misused power in his absence.
One interesting plotline could be Ash coming back with a new and powerful Pokemon team, ready to take on new challenges in the Pokemon world. Maybe he has a super rare Pokemon that no one has seen before.
One interesting plotline could be the restoration of the Targaryen dynasty in a more just and prosperous way. Maybe the dragonriders use their power to end feudal oppression. Another could be the discovery of new dragon eggs and the training of new dragonriders, which would bring a new dynamic to the story.
One popular plotline is the Ice Queen's journey to find true friendship. In many fanfictions, she starts off as a lonely and misunderstood character. Another plotline could be her attempts to gain more power in the Land of Ooo, which often leads to conflicts with other characters like Finn and Jake. Some fanfictions also explore her past and how she became the Ice Queen in the first place.
One possible plotline could be Marceline dealing with the challenges of pregnancy while still having to face the usual threats in the Land of Ooo. Maybe she has to rely on her friends like Finn and Jake more than ever for protection. Another could be exploring the relationship between her and the father of the baby, whoever that might be in the fanfiction, and how they navigate this new chapter in their lives.
Perhaps there could be a plot where evil humans form a secret organization. Their goal is to brainwash Pokemon to fight against other Pokemon and trainers. This would lead to a lot of chaos and destruction in the Pokemon world, and the story could focus on how the heroes uncover this plot and save the day.