Perhaps in the fanfiction, Arnold discovers a portal in his world that leads to Gravity Falls where he meets Dipper. Kryptos could be some kind of code or entity that exists in both their worlds. They start to investigate it together, facing challenges like strange creatures and obstacles set by the mystery of Kryptos. As they dig deeper, they learn more about each other's worlds and themselves.
Arnold would likely be curious about Dipper's world in Gravity Falls. He would ask a lot of questions, and Dipper, being the adventurous type, would be happy to share. They might start off a bit cautious around each other, but as they work together on the Kryptos mystery, they would grow to trust and rely on one another.
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.
A great idea for a Hey Arnold! married fanfiction could be Arnold and his wife starting a community project together. They could work on renovating an old building in Hillwood into a community center. This would show Arnold's altruistic nature continuing into his married life.
Maybe an OC who is an artist moves to the neighborhood. The OC could use their art skills to beautify the dilapidated areas around the boarding house. They might get into a friendly competition with Arnold to see who can make the most positive impact on the neighborhood. This could involve painting murals, organizing community art projects, and bringing the community closer together.
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 get paired for a school project and during the process, they start to truly understand each other's dreams and fears, which deepens their connection.
A possible plot could be that Arnold has a really bad day. First, he fails a big test at school, then he loses his favorite hat, and on top of that, his grandparents forget his birthday. All these disappointments pile up and he just snaps. He starts being mean to everyone, including his best friend Gerald. As the fanfic progresses, his friends could try to figure out what's wrong with him and help him through his tough time.
Another interesting plot could be that there's an ancient vampire curse in Gravity Falls, and Dipper gets caught up in it. He has to find a way to break the curse before it's too late, and in the process, he discovers more about the history of the town and its connection to vampires. This could involve teaming up with Mabel and other characters from the show to solve the mystery.
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 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.