It could be an old enemy of Finn and Jake who has come back to take revenge on their kids. Maybe the Ice King's descendants or something related to him.
There might be a new evil force that has risen in the Land of Ooo. This could be a powerful sorcerer who wants to control all the magic in Ooo. The kids, being Finn and Jake's offspring, are seen as a threat to this sorcerer's plans, so he becomes the main villain. He could be using dark magic to corrupt the land and its inhabitants, and the kids have to stop him. They would need to use their wits, the skills they learned from their dads, and maybe even make some new friends along the way to defeat this powerful foe.
Well, it could be about the kids going on their own crazy adventures. Maybe they inherit Finn's bravery and Jake's shape - shifting abilities and use them to save the Land of Ooo from a new threat.
One interesting fanfiction could be about Finn and Jake going on a quest to a mysterious new land that wasn't explored in the show. Maybe they encounter a new type of magical creature that challenges their friendship and they have to work together to overcome it.
Finn might initially go into denial. He could keep expecting Jake to just pop back up like he always does. Then he'd probably get angry, at himself or at whatever caused Jake's death. Eventually, he'd start to accept it and try to honor Jake's memory by being a better hero.
Maybe a new character, an ancient sorcerer who was once banished and now wants to use the Red Throne as a means to regain his power and take over Ooo. He could be a mysterious and powerful foe.
Princess Bubblegum could be a part of the fanfiction. She might be interested in the unique flora and fauna of the Canyon and could send Finn on a mission there. Marceline the Vampire Queen could also be added. Maybe she has some history with the Canyon and her presence could add an element of mystery and danger.
The Lich could be a great antagonist. He might frame Finn as part of his plan to spread chaos and death in Ooo. Since the Lich is always looking for ways to disrupt the peace, using Finn as a scapegoat would be a clever move for him.
It could also be a corrupted version of one of Fionna's friends. For example, if Cake were to be turned evil by some dark magic. Cake would know all of Fionna's weaknesses and could use that knowledge against Finn in his Fionna form, making her a formidable and tragic antagonist.
One interesting fanfiction could be about Finn and Jake going on a new, never - before - seen adventure in a strange magical land. Maybe they encounter a new type of magical creature that challenges their friendship and they have to work together to overcome it.
It could be an evil time - traveling wizard. He's jealous of Danny's powers and wants to steal them. He uses his magic to create all sorts of obstacles for Danny and Sam as they travel through time, trying to prevent them from getting back to their own time.
You can start by checking out popular fanfiction websites like Archive of Our Own. Just type in 'Adventure Time Finn x Jake' in the search bar and you'll get a list of relevant stories. Look for ones with high ratings and a lot of positive reviews.