In some Adventure Time Dark Finn fanfiction, a possible plot is that Dark Finn discovers a hidden power within his darkness. He then has to decide whether to use it for good or be consumed by it. This could lead to a series of battles, both internal and external. He may also encounter ancient beings that have knowledge about his condition and offer him different paths to take, like fully embracing the darkness or fighting to return to his old self. His relationships with characters like Jake, Princess Bubblegum, and Marceline would be severely strained, and exploring how he tries to repair or further damage those relationships could be a major part of the story.
One idea could be Dark Finn's struggle with his new identity. He might have to face his former friends who are now scared of him. Another could be his journey to find a way to reverse his dark state, leading him through strange and dangerous lands.