Maybe Simon could discover a hidden side - effect of his return. Like there are parts of the Ice King's magic that are still within him, but in a different form. This could lead to him having strange dreams that hint at a greater mystery, perhaps related to the origin of the Ice Crown. He then has to go on a quest to figure it all out.
One idea could be Simon's struggle to fully adapt back to his old self. He might have flashbacks of his Ice King days that cause him confusion and inner turmoil. For example, he could accidentally freeze things when he gets stressed, which would create interesting situations as he tries to control this new - old power.
One idea could be exploring their adventures in a new, uncharted part of the Land of Ooo. Maybe they find a magical artifact that has the power to change the past, which could bring back Simon's old self for a while. Another could be them getting stuck in a time - loop where they keep repeating a particular day and have to figure out how to break free.
There could be a fanfiction where Simon tries to reverse the effects of the crown in a more magical and mysterious way. It could involve him going on a quest to find a long - lost spell that might help him. This would add a lot of adventure and intrigue to the story.
It could be a remnant of the Ice King's past enemies. Maybe one of the magical creatures that the Ice King used to annoy or fight against now sees Simon as a threat. Since Simon has the knowledge and power of the Ice King, this creature wants to stop him before he becomes too powerful again.
One idea could be Shoko discovers a new, hidden realm in the Land of Ooo. She teams up with Finn and Jake to explore it and uncovers ancient secrets that could change the whole world of Adventure Time.
Well, a plot could revolve around the return of some old foes in a new form in the future. For instance, the Lich could come back as a more insidious threat that infects the minds of the future inhabitants of Ooo. Also, we could explore the idea of Finn's sword having new powers or a different significance in the future. Maybe it's the key to unlocking a long - forgotten dimension that holds the secrets to saving Ooo from a new impending doom.
One idea could be a new student arriving at Adventure Time Academy who has a mysterious power that no one has seen before. This new character could disrupt the existing social dynamics among Finn, Jake, and their friends.
Maybe Marceline the Vampire Queen teams up with a Dark - type Pokémon in a fanfiction. Since she has a somewhat dark and mysterious personality, it would be interesting to see how she interacts with a Pokémon that shares those traits. For example, she could form a bond with a Umbreon and they could go on adventures together.
One idea could be exploring Finn's family background more deeply. Maybe he discovers a long - lost relative who has a unique power. Another could be about Jake's pups and how they grow up in the Land of Ooo, facing different challenges as a family. Or perhaps a story about Princess Bubblegum's family tree and how it affects her rule in the Candy Kingdom.
One idea could be that the Water Princess discovers a hidden underwater kingdom that has a connection to the Land of Ooo. She has to team up with Finn and Jake to protect it from an evil sea creature.
One idea could be that the OC is a new traveler in the Land of Ooo. They accidentally save the lives of several main characters' love interests. As a result, these characters become attracted to the OC and form a sort of harem around them. For example, Princess Bubblegum's scientific assistant could be among them, drawn to the OC's unique knowledge from another land. Another plot could involve an OC who has a special power that is needed to save the harem members from an ancient curse. This would create a lot of bonding and relationship - building moments.