Adventure is also a big theme. The characters embark on new quests in unfamiliar settings. For instance, if Naruto is in the Mushroom Kingdom, he has to figure out how to deal with Goombas and Koopas while still maintaining his ninja skills. And for Mario in the Hidden Leaf, he has to adapt to the more dangerous and stealth - based ninja world. There's also the theme of power, as they combine their unique powers to face challenges.
It could be Bowser who has somehow gained the power of a powerful ninja jutsu. He uses this new - found power to take over both the Mushroom Kingdom and start invading the Naruto world. His fire - breathing ability combined with ninja techniques would make him a formidable foe for Naruto and Mario.
A great plot could involve a new threat that is a combination of the dark forces from both universes. Naruto and Mario find themselves in a strange new dimension where the rules of both their worlds blend. They must learn to trust each other and combine their unique skills. For example, Mario's invincibility star could enhance Naruto's speed and strength during battles, and Naruto's sage mode could help him sense hidden traps in the Mushroom Kingdom like he does in his own world.
A really creative one is 'Naruto and Mario: The Fusion Quest'. This fanfiction has a magical device that fuses Naruto and Mario into one being. This new entity has to use the combined powers of both to stop an ancient evil that threatens both the Mushroom Kingdom and the Hidden Leaf Village. It's a wild concept that makes for an exciting read.
Once upon a time, Mario found himself in the magical world of MLP. He was quite confused at first, with all the colorful ponies and floating castles. But being the brave hero he is, he decided to explore. He met Twilight Sparkle, who was intrigued by this strange new being. Mario showed her his jumping skills, and Twilight was amazed. They then went on an adventure together to find a way to send Mario back to his world, facing all kinds of challenges along the way, from evil magic spells to tricky terrains.
Once upon a time, Mario found himself in the world of Splatoon. He was confused at first, looking at all the inklings and octolings. But being the brave hero he is, he decided to join in the Turf War. He jumped around, using his plumber skills to dodge the ink blasts. Mario's red cap stood out in the colorful ink - covered battlefield. He soon made friends with some inklings who taught him how to use the ink guns. Together, they fought against a group of octolings and won the Turf War. It was a strange but exciting adventure for Mario.
Another theme is friendship. When Mario meets characters from different franchises, they often have to work together. In a Mario and Zelda crossover, Mario and Link may start off as strangers but quickly become allies through their shared goal of defeating the evil force. They learn about each other's strengths and weaknesses and form a bond. And in a crossover with Pokémon, Mario might make friends with Pokémon trainers and Pokémon alike.
Once upon a time, Michael Rosen found himself in the Mushroom Kingdom. Mario was on a mission to rescue Princess Peach as usual. Michael, being his curious self, decided to tag along. He started telling his famous stories to Mario as they hopped over the mushroom platforms. Mario was initially confused but soon found the stories quite interesting and it gave him new energy to face the challenges ahead. Together, they defeated Bowser in a rather unconventional way. Michael used his words to outwit Bowser while Mario did the physical fighting.
To start a Mario and real - life crossover fanfiction, consider the 'why'. Why is Mario coming to our world? Is it to save it from a new threat that has somehow crossed over from his world? Or is it simply an accident? You could start with a dream sequence where a character in the real world dreams of Mario, and then wakes up to find him actually there. This allows for an interesting start as the character has to quickly come to terms with the fact that their dream has become a reality.
In a Mario and real - life crossover fanfiction, Mario could find himself in a school. The kids are amazed and start following him around. Mario teaches them about his world, like the different power - ups and how to face challenges bravely. He might even turn the school playground into a mini - Mushroom Kingdom for a day, with the kids helping him create makeshift Goombas and Koopas out of cardboard and toys.