One idea could be that Bella discovers a new power that she and Edward have to figure out how to control together. It adds a new layer to their relationship and the overall Twilight -esque world they inhabit.
Another plot idea could be that Bella starts to have strange dreams during the honeymoon. These dreams seem to be prophetic, showing her glimpses of their future together and some potential threats. Edward tries to comfort her and figure out what these dreams mean, which could lead to a deeper exploration of their relationship and the Twilight world.
A possible plot could be that there is a threat to the vampire community that only Bella and Carlisle, with their combined skills and knowledge, can solve. As they work together, they start to see each other in a different light. Carlisle might see Bella not just as a young vampire but as a strong and intelligent equal, and Bella could gain a deeper respect for Carlisle's wisdom and experience. During this time, their relationship could evolve in unexpected ways, like a deeper friendship that is on the verge of something more, but they are hesitant due to the age gap and their different roles in the vampire family.
Some fanfictions have the idea of Bella having special powers other than just being a shield. For example, in one she can communicate with animals, which changes the way she interacts with the vampire - werewolf world.
Perhaps in the fanfic, there could be a situation where Bella and Charlie have to team up to face a new threat in Forks. This could strengthen their bond and make them see each other in a different light. They might have to rely on each other's unique skills to overcome the danger.
One idea could be that Edward is a criminal on the run and Bella, a journalist, is trying to expose his crimes but ends up falling in love with him. Another could be Edward as a criminal mastermind who recruits Bella into his world of crime. Maybe Edward is wrongly accused of a crime and Bella helps him prove his innocence while they face many dangerous situations together.
One interesting plot idea could be a love triangle where both Edward and Emmett are vying for Bella's attention in a more complex way than in the original story. Maybe there are external forces at play that make Bella question her feelings for both of them.
Well, perhaps in this fanfiction, the shower is a place where they find a magical or symbolic element. It could be that the water has some sort of special property that helps Edward control his thirst for Bella's blood in a new way. Or, the plot could revolve around them getting into an argument in the shower, which then leads to a passionate reconciliation as they realize how much they mean to each other and how they need to communicate better.
Perhaps in the fanfiction, Bella is also in prison for some unjust reason. Edward, being a vampire, has to use his special abilities secretly to protect her from the other inmates. He could be using his mind - reading skills to anticipate threats.
A plot could involve Renesmee's love life. Since she is a special being, finding a suitable partner would be difficult. It could be a story of her falling in love with a human who is initially afraid of her but gradually comes to accept her. Edward and Bella would be there to guide her through this new experience, while also being protective. Or perhaps a plot where Renesmee discovers a hidden power within herself that has been passed down from an ancient vampire ancestor, and she has to learn to control it with the help of Edward and Bella.
Maybe a plot where Renesmee has to face some sort of vampire - human conflict that her parents once faced, but in a different context. Since she is part - vampire and part - human, she could be a bridge between the two worlds and strive to bring peace. She could use her understanding of both sides to defuse tense situations.