One idea could be a story where a new housemate has a secret superpower that they have to keep hidden while in the Big Brother house. Another could be a fanfiction about a past housemate returning in a different role, like as a guest host. Or perhaps a story where two housemates form an unlikely alliance based on a shared love for a particular type of music that they discover in the house.
Well, you could write a big brother fanfiction where there's a mystery in the house. For example, strange things start happening like items going missing or mysterious messages being left. The housemates have to work together to figure out what's going on. Or you could focus on a love triangle between three of the housemates, with all the drama and jealousy that comes with it. Also, a fanfiction about a housemate who is actually a spy from the production team could be really interesting.
In this type of fanfiction, a possible plot is that the Death Eaters and the Order of the Phoenix members are forced to live together in a 'Big Brother' style house. They have to follow certain rules set by the magic of the house. There are tasks related to both dark and light magic. The characters have to balance their own beliefs and goals while also trying to win the overall competition. This creates a lot of tension and interesting character interactions as they all have very different motives.
A great plot idea could be Percy having to deal with the jealousy of other demigods because of his special status as the 'brother' of the Big Three. This could lead to some tense relationships and even battles within Camp Half - Blood. Or perhaps Percy is forced to make a difficult choice between loyalty to his mortal friends and family and the demands that come with his connection to the Big Three. Maybe there's a story where Percy uncovers a secret power that only he, as related to the Big Three, can access, but it comes with a great cost.
One idea could be that Big Brother is a protector in the BioShock world, guiding Little Sister through the dangerous and strange underwater city. He helps her avoid the Splicers and find the ADAM she needs in a more ethical way, like by using his combat skills to take out the Big Daddies first so she can harvest safely.
One idea could be that the phantom brother has special powers like invisibility and uses them to protect the main character from hidden threats. He might be a ghostly figure from a past era, and his presence is only known to the main character, adding an element of mystery to the story.
One plot idea could be that Baby Loki gets into some sort of magical mischief in Asgard while Thor tries to keep him out of trouble. For example, Loki might accidentally turn all the flowers in the palace gardens into snakes, and Thor has to figure out how to reverse it before the other Asgardians find out.
One idea could be exploring the daily adventures of a big baby in a magical world. Maybe they have special powers despite being in a baby - like state. Another could be about how the big baby forms unique friendships with other characters who are initially puzzled by this big baby concept.
One idea could be a crossover with 'Frozen'. Brother Bear's Kenai could meet Elsa and they could learn from each other's experiences with magic and transformation. Elsa could help Kenai understand his own new form better, and Kenai could teach Elsa about the spirits in his world.
One idea could be exploring the backstories of the brothers in Fairy Tail. For example, delving into their childhood and how they developed their magic skills. Another could be a story where the brothers have to overcome a great danger together, strengthening their bond in the process.
One idea could be a fanfiction where the brothers in the 'Brother Game' find themselves in a mysterious part of the Big Valley. They could encounter strange creatures and have to work together using their unique skills from the game to survive.
One idea could be exploring the backstories of the Big Chill Babies. Maybe they have unique powers that are yet to be fully discovered. Another could be a story about their first encounter with other alien species and how they interact.