One idea could be a story where Jin and Mugen have to team up against a powerful common enemy. They start off not getting along well, as is their nature, but as they face challenges together, they gradually learn to respect each other's skills and develop a deep bond.
A Jin x Mugen fanfic could also explore their backstories. Maybe there was a time in their past when they unknowingly crossed paths. The story could show how their actions at that time, even without them realizing it, had an impact on who they are now and how they interact in the present.
One idea could be a story where Mugen and Jin are forced to team up against a powerful new enemy that threatens their town. They have to put aside their differences and use their unique fighting styles in unison.
I'm not very familiar with specific 'jin x mugen yaoi fanfic' stories. However, generally, you can try to search on some fanfic platforms like Archive of Our Own. There might be a variety of works created by fans in that category.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, and there's a good chance you'll find some great Jin x Mugen ones there. Just use the search function to narrow it down.
First, you need to have a clear idea of the relationship between Jin and Mugen. Think about their personalities, how they interact, and what kind of story you want to tell. For example, is it a story about their first meeting or a more complex relationship development.
One idea could be exploring the origin of Alien X's powers. Maybe there was an ancient alien race that bestowed these powers and the story could delve into that backstory.
One idea could be a story where Spiderman has to team up with his greatest enemies to fight a new, more powerful threat from outer space. This would force him to put aside his differences and find common ground with the likes of Doctor Octopus and Green Goblin.
Another interesting concept could be a story about Naruto visiting Starfire's world. He could be amazed by the different technology and culture there, while Starfire is intrigued by Naruto's ninja skills and his chakra.
Perhaps they could meet at a music festival. Asha is lost among the crowds and Starboy helps her find her way. This leads to them spending more time together, sharing their dreams and aspirations in the music industry. They then decide to form a duo for a special performance at the festival.
One idea could be that Shrek and Shaggy meet in a magical forest that's a cross between Shrek's swamp area and the spooky woods where Shaggy often has his adventures. They could team up to find a special ingredient for a potion that would solve a problem in both of their worlds, like a drought in Shrek's land and a monster invasion in Shaggy's.
One idea could be a story where Luffy and Sanji get separated from the crew during a battle and have to rely on each other to survive on a strange island. They could discover new types of food that only Sanji can cook and Luffy can't resist, leading to some comical and heartwarming moments.