One plot idea could be that Stanford Pines discovers a portal to Star Butterfly's dimension. He teams up with Star to stop an evil force that is threatening both their worlds. Maybe it's an ancient entity that has the power to disrupt magic in Mewni and the strange anomalies in Gravity Falls.
A plot could involve Star Butterfly getting lost in Gravity Falls. Stanford Pines takes it upon himself to help her get back home. Along the way, Star's magic causes some chaos in the town, and they have to deal with local mysteries and the nosy townsfolk. At the same time, Stanford is learning about Star's world and magic, which could give him new insights for his research.
You might want to look for fanfics on Wattpad as well. There could be some hidden gems. A possible scenario in a 'Stanford Pines meets Star Butterfly' fanfic could be that Star's friends from Mewni visit Gravity Falls. Stanford, being the curious and knowledge - hungry person he is, would be intrigued by their magic and might try to study it. This could lead to all sorts of comical and exciting situations as the two sets of characters learn from each other.
Often, there are themes related to his past and how it affects his present actions. For example, his time in the other dimension and how that shapes his character in the fanfiction. Also, themes of self - discovery are quite common. As he's a complex character, writers might explore how he further discovers himself in different situations in the Lemon fanfiction.
A really interesting angle could be a story that focuses on how Star's friends and family react to her mewberty transformation. Do they support her? Are they afraid? You could show the different perspectives of characters like Moon, River, and Pony Head as they try to deal with this new version of Star.
Another aspect could be the character relationships. Maybe a crossover could lead to unique friendships or rivalries. For instance, characters who are similar in some ways but from different worlds might initially clash before understanding each other better. In a star butterfly crossover fanfic, we could also see how different cultures in their respective universes blend together, like their styles of clothing, food, and social norms.
Well, it could be that '13' is an alien entity that crash - lands near a river on Earth. The fanfic could then explore how this alien, which is completely out of its element, tries to adapt to the new environment around the river. It might encounter local wildlife, humans, or other strange phenomena related to the river. Another plot could be that '13' is a cursed object or person, and the river has the power to break the curse. So the story would revolve around '13' trying to reach the river and overcome the obstacles in the way to be freed from the curse.
One plot idea could be a diplomatic mission. Imagine Naruto characters being sent as representatives to the Star Wars galaxy. They would have to learn about the different cultures and politics there, and use their ninja skills in negotiation and understanding. For example, Naruto himself could use his ability to understand others' pain to bridge gaps between warring factions in the Star Wars universe.
You can try searching on fanfiction websites like Archive of Our Own (AO3). There are often a lot of fan-made stories there, and you might be able to find some great Lemon Stanford Pines fanfiction by using the search function and filtering the results.
One idea could be Star's journey to discover a new type of magic that has been lost in her family's history. It could involve her exploring ancient libraries and facing challenges along the way.
In a fanfic, Madama Butterfly could lose her memory. Bayonetta then has to help her regain it while also protecting her from enemies who see this as an opportunity to attack. Bayonetta could show Madama Butterfly old memories through their shared magic, and in the process, they both learn more about each other and their own powers.
Well, they could go on an amazing adventure together. Percy could use his water - controlling powers and Star could use her magic wand. Maybe they'd fight some mythical creatures that are a mix of the Greek mythology ones Percy knows and the magical ones from Star's dimension.