Perhaps Six has a change of heart and tries to understand the Lady's motives more deeply. She could discover that the Lady was once a victim herself and was forced into her role, leading to a complex relationship development in the fanfiction.
A plot could revolve around Six getting lost in a different part of the world in Little Nightmares and the Lady is the only one who can guide her back. Along the way, they start to see each other in a new light and share their own stories of survival and fear.
Perhaps Mono and Six get separated in a really dangerous area. Mono has to fight his way through hordes of the faceless enemies to find Six again. Meanwhile, Six is using her wits to stay alive and find a way back to Mono. This would create a lot of tension and suspense in the fanfiction.
A different plot could be centered around a betrayal. Maybe Seven is forced to betray Six due to some external influence, like a mind - controlling force in the Maw. Six then has to decide whether to forgive Seven and try to save him from this control, or leave him behind and continue on her own journey. This would create a lot of tension and drama in the story.
For Six, you can explore her past more deeply. Maybe show flashbacks of her life before the events of the game that led her to be so resourceful and resilient. As for the Lady, you could give her more humanizing traits, like a hobby or a soft spot for something.
One idea could be a story where Six and Seven form an unbreakable alliance to face the terrors of the Maw together. They could discover hidden passages and use their unique skills to outwit the monsters. For example, Six's ability to be stealthy and Seven's strength could be combined.
One idea could be exploring the backstory of the Thin Man. Maybe he was once a normal child in the world of Little Nightmares but something happened to turn him into that terrifying figure.
Another fanfiction could focus on a different ending for Six. Maybe in this fan-made story, Six doesn't have to go through all the horrors alone and she teams up with other characters in a more positive way, changing the whole course of her journey.
Since there's no clear information on a specific 'Little Nightmares Six Story', we can only speculate. It might expand on the themes of fear, isolation, and the struggle for survival that are prevalent in the series. It could introduce new areas of the world, like a different part of the Maw or a completely new and equally horrifying location. Six might also face moral dilemmas as she continues her journey, which would add more depth to the story.
A cool plot could be a character swap. Imagine Frisk from Undertale in the world of Little Nightmares. Frisk's determination would be put to the test in the nightmarish setting of Little Nightmares. They would have to face the Janitor and the Twin Chefs just like Six does. Meanwhile, Six could be in the Underground, dealing with Flowey and Toriel. The contrast in the two worlds' aesthetics and challenges would make for an exciting story.
An OC could be a former cook in the Maw who has a change of heart. The fanfiction could explore their journey of redemption as they help the other children. They might have knowledge of the Maw's layout and use it to their advantage.
Fanfiction about Mono and Six in Little Nightmares can be really interesting. It often explores their relationship in more depth. Some fanfics might focus on their adventures together before or after the events in the game. It can show different sides of their characters, like Mono's determination and Six's survival instincts.