A possible plot could be that Reverse Dipper has a task to complete in Gravity Falls, and Mabel accidentally gets involved. They have different goals at first. Reverse Dipper is focused on his own mission which might be more serious and goal - oriented, while Mabel is just along for the ride and looking for some new experiences. But as they face various obstacles together, they start to understand each other better and become friends. They might even find a way to combine their goals in the end.
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 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.
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 a story where Baby Dipper discovers a magical creature in the forest and Mabel helps him keep it a secret from the adults. They go on adventures to find food for the creature and learn about its magical abilities.
One idea could be that they start off as rivals but then find themselves in a situation where they have to team up to solve a mystery in Gravity Falls. As they work together, they begin to understand each other better and a strange bond forms between them.
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 a story where baby Dipper and Mabel discover a magical creature in the woods near their home in Gravity Falls. The creature is lost and they decide to help it find its way back. Another could be a fanfic about their first day at daycare in Gravity Falls and all the strange things that happen there. Maybe they meet other kids with unique abilities or find a secret part of the daycare.
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 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.
One interesting Dipper and Mabel fanfiction could be about their further adventures in Gravity Falls after the events of the show. Maybe they discover a new mystery that involves an ancient creature from the forest. Another could be a story where they swap personalities for a day and how they deal with it. And there might be a fanfic that focuses on their relationship with other characters in a different way, like how they interact with the gnomes in a more complex plot.
Maybe the fanfic could start with Mabel finding a lemon - shaped amulet. When Dipper touches it, they get transported to a parallel Gravity Falls where everything is lemon - themed. They have to find a way back while dealing with the wacky versions of the townspeople. This would allow for a lot of creative scenarios and character interactions.