Maybe they get trapped in a game world where the rules keep changing randomly. They then have to figure out how to adapt and find a way out while also dealing with the strange inhabitants of that world. This could lead to some really interesting character development and interactions between them.
The main characters are, of course, Captain N and the Game Master. Captain N is often portrayed as a heroic figure, coming from our world but having the ability to enter and interact with various game worlds. He uses his wits and some special powers to solve problems in those worlds.
One idea could be a competition among different wizards to claim the title of master of death in a more complex and dangerous magical arena. They might have to face challenges that test not only their magical skills but also their moral compasses.
Perhaps a fanfiction where Tsubasa time - travels back to the early days of football in Japan. He meets the pioneers of the sport in his country and uses his modern skills and knowledge to inspire them and help develop the game even faster than it actually did. He could also face challenges in this different era, like dealing with more primitive equipment and different rules interpretations.
Another idea is that there's a villain who can manipulate time and space. He sends Captain America to the Naruto world during a crucial time in Naruto's story, like right before a big battle. Captain America's leadership and battle experience help Naruto and his friends come up with a better strategy. And in the process, they learn from each other's values and fighting styles.
One plot idea could be that 7 of 9 and Captain Janeway have to work together on a dangerous mission to save the ship. They start off with a bit of a rocky relationship due to their different outlooks, but as the mission progresses, they learn to respect and rely on each other more. Another could be a story set in an alternate universe where they are friends from the start and have to deal with a new alien threat that only their combined skills can defeat.
One idea could be Captain Marvel bringing back some alien tech that the Avengers have to figure out how to use. It could lead to new powers for some of the members or a new way to defend Earth.
One idea could be exploring the backstory of the captain before the promise was made. Maybe there was a tragic event in their past that led them to make such a solemn vow. Another could be a situation where the captain has to break the promise to save their crew, and then deal with the moral and emotional consequences.
One idea could be a crossover where the Naruto characters interact with the Shinigami captains in a battle against a common enemy. For example, Naruto's unique jutsu combined with a Shinigami captain's zanpakuto abilities would make for some epic fight scenes.
One plot idea could be that Tinkerbell is sent on a mission by the fairies to steal a magical item from Captain Hook. But during the process, she discovers that Hook has a tragic past that led him to his villainous ways. This makes her question whether he's truly all bad.
One plot idea could be an OC who is a scientist during World War II and teams up with Captain America. They could work on some top - secret projects together. Another is an OC who is a foreign agent sent to assess Captain America's abilities and ends up becoming his ally. Maybe an OC from the future who travels back in time to help Captain America prevent a major catastrophe that wasn't in the original timeline.