Bill Cipher would be an interesting character to involve. If he shows up, he could be even more manipulative towards Reverse Dipper and Mabel. His chaotic nature could create a lot of conflict and drama. Reverse Dipper might be more vulnerable to Bill's tricks or perhaps more resistant in some unexpected ways. And Mabel would have to use her wits and kindness to try to counter Bill's evil plans.
One idea could be that Reverse Dipper, who might have different characteristics in this reversed world, meets Mabel and they initially have a misunderstanding because of their different ways of thinking. Maybe Reverse Dipper is more cautious while Mabel is her usual exuberant self. They could go on an adventure where Mabel tries to teach Reverse Dipper how to have more fun and let loose.
One common theme could be role - reversal in their personalities. For example, Dipper might be more carefree and Mabel more cautious. Another theme could be exploring a different kind of relationship dynamic between them, like a more romantic - friendship hybrid.
One potential antagonist could be Bill Cipher from Reverse Dipper's dimension. He might have different motives or be more powerful in that realm and see the meeting of the two Dippers as a threat to his plans.
One place to start looking for 'reverse Dipper x Mabel fanfiction' could be Archive of Our Own (AO3). It has a vast collection of fan - created works, and you can use the search function to specifically look for stories with this pairing.
There's also 'The Switched Roles'. Here, the author takes the concept of reverse Mabel x Dipper quite literally. Mabel is more of the rational, protective one while Dipper is the carefree and creative one. It's a fresh take on their relationship and makes for an engaging read.
One idea could be that Reverse Dipper, coming from his alternate dimension, initially confuses the regular Dipper. Maybe they start by arguing about who is the'real' Dipper. Then, they could team up to solve a mystery that involves both their dimensions.
You can start by searching on major fanfiction platforms like Archive of Our Own (AO3). Just type in'reverse Mabel x Dipper' in the search bar, and it will show you a list of relevant stories. AO3 has a wide range of fanfiction, so you're likely to find many interesting ones there.
The relationship would be filled with new emotions. There would be moments of stress, like when the baby is sick and they don't know what to do. But there would also be moments of great joy, like when the baby takes its first steps. Through all these experiences, they would understand each other better and their relationship would evolve into a more complex and loving one.
In such a fanfic, Bill Cipher might try to trick Dipper and Mabel. He could offer them false knowledge or tempt them with strange powers. Maybe Dipper would be the first to suspect Bill's motives, while Mabel might be more easily swayed by his charm at first.