The Ice King can be a key character too. Maybe he tries to kidnap Minerva for his own strange reasons, thinking she can be his new princess. Marceline the Vampire Queen could also be involved. She might be Minerva's ally or foe depending on the plot of the fanfiction. There are also other minor characters from the Adventure Time series that can play important roles in Minerva - related fanfictions.
Some might. If the fanfiction is positive and fun, they could get a kick out of it. Just like how we enjoy seeing positive interpretations of our favorite things.
Obviously, Shoko is a main character. She is often depicted as a tough and resourceful girl with her own set of skills. Another important character could be Finn. Finn might cross paths with Shoko and they could either become friends or rivals depending on the plot. Jake, Finn's loyal dog, could also play a role, perhaps helping or hindering Shoko's plans.
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.
Finn and Jake are still really popular even in genderbend fanfiction. Their friendship is iconic, and seeing how it changes when their genders are swapped is fascinating to many fans.
Well, the si gamer is obviously one of the main characters. Then there could be Finn and Jake who might interact with the si gamer. Finn with his adventurous spirit and Jake with his shape - shifting abilities would make for interesting interactions with the si gamer.
Finn would be a great character to pair with the Lich. Finn is the hero, so the contrast between his good - natured self and the Lich's evil would create a lot of interesting story possibilities. They could have some epic battles or, as I mentioned before, an unexpected alliance.
The Ice King can also be a part of adventure time bandit princess fanfiction. He might try to capture the Bandit Princess for his own strange reasons, like wanting her to be his ice princess. His bumbling nature and ice powers create comical and challenging situations in the story.
Princess Bubblegum's future self could be a main character. She might be leading a resistance against a new tyranny in Ooo. Also, the Candy Kingdom could have new heroes from within its walls, like a young gumdrop who discovers a hidden power within herself and becomes a key figure in the future battles. Additionally, a future version of Marceline could be involved, perhaps she has become a mentor to the new generation of adventurers.
The main characters are Leo and Blake, of course. They are likely to be the ones driving the story forward, getting into all sorts of adventures in the Adventure Time universe.