Some Mario fanfics revolve around the relationships between the characters. A really good one could be about Princess Peach and her friendship with Toad. It could detail their adventures when Mario and Luigi are away, like how they defend the Mushroom Kingdom from smaller threats on their own.
One interesting story could be about Mario and his friends having a special adventure during their time at the camp. Maybe they discover a hidden part of the camp with magical mushrooms that give them new powers.
One idea could be that Naruto gets transported to the Mario world. He might use his ninja skills to help Mario and Luigi in their battles against Bowser. For example, Naruto could use his Shadow Clone Jutsu to confuse Bowser's minions while Mario goes for the power - up.
You could write a Paper Mario fanfic that involves time travel. Mario goes back in time to prevent a great disaster that happened in the past of the Paper Mario world. This could lead to him interacting with past versions of characters and changing the course of history in unexpected ways. Or, a fanfic about a new villain that only exists in the paper realm and has unique powers that Mario has to figure out how to defeat.
One idea could be a racing tournament where Mario and Sonic team up against some new, powerful villains. They have to combine their unique skills, like Mario's jumping and Sonic's speed, to overcome the challenges of the track which is filled with traps from both of their worlds.
One idea could be a story where Mario and Lemon (assuming it's a character) go on an adventure through a new and mysterious world in the Mushroom Kingdom. They could encounter new enemies and have to use their unique skills to overcome challenges.
One interesting Mario game story is in 'Super Mario Bros.' where Mario has to rescue Princess Peach from Bowser. He jumps over obstacles, defeats Goombas and Koopa Troopas, and travels through different worlds like the Mushroom Kingdom to reach Bowser's castle.
One idea could be a racing tournament where Mario, Sonic and their friends compete. It would be a high - speed adventure with crazy power - ups from both universes. Maybe Mario could use a Sonic - inspired speed boost item, and Sonic could try out a mushroom to grow big for a short time.
One interesting plot could be Bowser and Mario having to team up against a greater evil. This would flip their usual dynamic on its head and lead to some really fun character interactions.
One idea could be that Pauline gets kidnapped by Bowser and Mario has to go on an extra challenging adventure to save her. He might face new types of enemies and obstacles along the way.
One interesting Dark Mario fanfiction could be about Dark Mario's origin. Maybe he was created from a dark magic experiment gone wrong in the Mushroom Kingdom. Another could involve his adventures in a parallel universe where he's the main villain and has to face off against a heroic Luigi. There are also those that explore his relationship with other characters like Princess Peach, but in a much darker and more complex way than in the original games.